High Performance Audio Power Amplifiers for Music Performance and Reproduction

High Performance Audio Power Amplifiers for music performance and reproduction

Newnes An imprint of Butterworth-Heinemann Ltd Linacre House, Jordan Hill, Oxford OX2 8DP A division of Reed Educational and Professional Publishing Ltd OXFORD MELBOURNE

BOSTON NEW DELHI

First published 1996 © Ben Duncan 1996

JOHANNESBURG SINGAPORE

Reprinted with revisions 1997 © B. D. 1997

All rights reserved. No part of this publication may be reproduced in any material form (including photocopying or storing in any medium by electronic means and whether or not transiently or incidentally to some other use of this publication) without the written permission of the copyright holder except in accordance with the provisions of the Copyright, Designs and Patents Act 1988 or under the terms of a licence issued by the Copyright Licensing Agency Ltd, 90 Tottenham Court Road, London, England W1P 9HE. Applications for the copyright holder's written permission to reproduce any part of this publication should be addressed to the publishers TRADEMARKS/REGISTERED TRADEMARKS Computer hardware and software brand names mentioned in this book are protected by their respective trademarks and are acknowledged. British Library Cataloguing in Publication Data A catalogue record for this book is available from the British Library ISBN 0 7506 2629 1 Typeset by P.K.McBride, Southampton Printed and bound in Great Britain

High Performance Audio Power Amplifiers for music performance and reproduction

Ben Duncan,

A.M.I.O.A., A.M.A.E.S., M.C.C.S

international consultant in live show, recording & domestic audio electronics and electro-acoustics.

Foreword Ben Duncan is one of those rare individuals whose love and enthusiasm for a subject transcends all the usual limits on perception and progress. In fact, without the few people of true independent spirit, progress in the world would be swamped by the xylocaine of vested interest, narrow attitude and corporate monoculture. Amongst my early experiences of Ben Duncan’s thinking, many years ago, were his contention that electronic components have qualitative audio properties and his recommendation that we listen to the sound of capacitors of various dielectrics. The outcome was the exclusive use of polypropylene capacitors in all Turbosound’s passive hi-pass networks. This is not only illustrative of the depth to which the man goes, but also his extensive seen and unseen influence on the whole audio community. He is an holistic thinker and I believe there are very few things in the Universe that he has not, at one time or another, considered having an effect on audio quality. Does he keep his flights of fancy and strokes of brilliance to himself? Not one bit of it. He communicates compulsively and in large quantities as anyone who has followed the general audio press for the last dozen or so years will tell you. A memorable early experience of power amplifiers was with the then relatively new transistor variety powering a P.A. I had built for the Pink Fairies, that was at the original Glastonbury in 1971. After the sixth failure of an HH TPA100, for no apparent reason, I was running out of working stock. On sitting down to consider the hopeless situation it became worse when I found the live soldering iron. My next immediate thoughts were about a change of career. Anyhow, the point of this sad little tale is that in those days power amplifiers were absolutely horrible things because despite the fact that they had somewhat puny voltage swings they were, nevertheless, always blowing up at the slightest opportunity and particularly in the hour before show time. These days things have progressed a long way and sound system operators bask in the luxury of equipment that is almost indestructible and capable of audio quality usually associated with esoteric hi-fi as well as delivering arc welding levels of power. I am extremely grateful to Ben that he has undertaken the Herculean task of collating all the relevant facts on, and to do with, power amplifiers ranging from the in depth assessment of household mains to determinations as to whether it actually sounds any good. The breadth of the book enables an average human to purchase or design power amplifiers knowing that all relevant information is at their disposal and as such this book should be considered a positive contribution to the sum total of mankind. I hope it has a similar effect on his bank balance.

Tony Andrews, Hoyle, Surrey March 1996

Contents Preface ....................................................................................................................... xi Acknowledgments ................................................................................................... xiii System of presentation ............................................................................................. xv

1

Introduction and fundamentals

1

1.0. 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 1.10

What are audio power amplifiers for ? ............................................... 1 What is the problem ? ......................................................................... 1 What is audio ? ................................................................................... 2 What’s special about audio ? .............................................................. 2 The ramifications of quality on audio ................................................ 3 Some different aims of sound reproduction ....................................... 3 About people and their hearing .......................................................... 4 Limits of a ‘objectivity’. Why listen ? ............................................... 5 Why are power amplifiers needed for audio ? ................................... 6 Music fundamentals ........................................................................... 8 Adjectives that describe sound ........................................................... 9

1.10.1 1.10.2 1.10.3 1.10.4 1.10.5 1.10.6

Tonal qualities .............................................................................................. 10 Broader tonal descriptors .............................................................................. 11 General sonic adjectives ............................................................................... 11 Dynamics ...................................................................................................... 12 Space ............................................................................................................ 13 Botheration or Abomination ........................................................................ 14

1.11 Nature and range of music (alias programme) ................................. 14 1.12 Bass and subsonic content ................................................................ 15 1.13 HF dynamics and ultrasonic content ................................................ 16 References and Further reading ................................................................... 18

2

Overview of Global Requirements

19

2.1 2.2

Common formats for power amps ................................................... 19 Loudspeakers .................................................................................... 21

2.2.1 2.2.2 2.2.3 2.2.4

Loudspeaker drive-unit basics ..................................................................... Loudspeaker sensitivity vs. efficiency ......................................................... Loudspeaker enclosure types and efficiencies ............................................. Loudspeaker configurations: a résumé ........................................................

2.3

The interrelation of components ...................................................... 32

21 25 26 27

2.3.1 What loudspeakers look like to the amplifier .............................................. 32 2.3.2 What speakers are looking for ..................................................................... 35 2.3.3 What passive crossovers look like to amplifiers ......................................... 38

2.4

Behaviour of power amps as voltage sources .................................. 40

2.4.1 2.4.2 2.4.3 2.4.4 2.4.5

Drive-unit power ratings after EIA/AES ..................................................... Output power capability requirements ......................................................... Loudspeaker vulnerabilities ......................................................................... High power, the professional rationale ........................................................ Active systems, power delivery requirements .............................................

2.5

Current delivery requirements .......................................................... 46

40 41 43 44 46

2.5.1 The low impedance route ............................................................................. 47 References and Further reading ................................................................... 48

3

The input port – Interfacing and processing 3.1

49

The Input........................................................................................... 49

3.1.1 Input sensitivity and gain requirements ....................................................... 49 3.1.2 Input impedance (Zin) ................................................................................... 52

3.2 3.3

RF filtration ...................................................................................... 58 The balanced input ........................................................................... 59

3.3.1 Balancing requirements ............................................................................... 59 3.3.2 Introducing Common Mode Rejection ........................................................ 60

3.4

Sub-sonic protection and high-pass filtering ................................... 63

3.4.1 Direct Coupling ............................................................................................ 65

3.5 3.6

Damage protection ........................................................................... 68 What are process functions? ............................................................. 71

3.6.1 3.6.2 3.6.3 3.6.4 3.6.5 3.6.6 3.6.7

Common gain control (panel attenuator) ..................................................... Remotable gain controls (machine control) ................................................. Remote control considerations ..................................................................... Compression and limiting ............................................................................ Clipping (overload) considerations .............................................................. Clip prevention ............................................................................................. Soft-Clip .......................................................................................................

3.7

Computer control .............................................................................. 80

71 74 77 78 79 79 79

References and Further reading ................................................................... 82

4

Topologies, classes and modes 4.1

83

Introduction ...................................................................................... 83

4.1.1 About topologies .......................................................................................... 84

4.2

Germanium and early junctions ....................................................... 87

4.2.1 4.2.2 4.2.3 4.2.4 4.2.5 4.2.6 4.1.8 4.1.9

Out of the vacuum-state ............................................................................... Push-pull, Transformer-coupled .................................................................. Sub-topology: the Darlington ...................................................................... Transformerless push-pull (transistor OTL) ................................................ Sub-topology: diode biasing ........................................................................ Complementary push-pull OTL ................................................................... Quasi complementarity: the faked match .................................................... Sub-topology: paralleling .............................................................................

87 88 89 91 91 92 94 94

4.3

Silicon transistors ............................................................................. 95

4.3.1 4.3.2 4.3.3 4.3.4 4.3.5 4.3.6

The Lin topology .......................................................................................... 96 Sub-topology: the long-tailed pair (LTP) .................................................... 98 Sub-topology: the Vbe multiplier (VbeX] ................................................... 99 Sub-topology: the triple (compound BJT) ................................................. 100 Sub topology: Dual supplies (+/–Vs) ........................................................ 102 Sons of Lin ................................................................................................. 104

4.4

True symmetry: the sequel ............................................................. 105

4.4.1 4.4.2 4.4.3 4.4.4

Later topologies .......................................................................................... 106 IC power ..................................................................................................... 108 The Op-Amp topologies ............................................................................. 110 Power cascades and cascodes ..................................................................... 112

4.5

Introducing bridging ....................................................................... 113

4.5.1 Bridging the bridge ..................................................................................... 117

4.6

Class-ification ................................................................................. 118

4.6.1 Class A ........................................................................................................ 118

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4.6.2 4.6.3 4.6.4 4.6.5 4.6.6 4.6.7 4.6.8

Class A alternatives .................................................................................... Class A sliding bias and ‘Π-mode’ ............................................................ ‘Super Class A’ ........................................................................................... Dynamic biasing and Stasis ....................................................................... Sustained plateau biasing ........................................................................... Class B and A-B ......................................................................................... Class A-B, developments and ameliorations .............................................

123 123 126 126 126 127 131

4.7

Introducing higher classes .............................................................. 136

4.7.1 Class G ....................................................................................................... 138 4.7.2 Class H ....................................................................................................... 141 4.7.3 G and H, the comparison ........................................................................... 143

4.8

Beyond analogue ............................................................................ 146

4.8.1 Class D ....................................................................................................... 147 4.8.2 ‘Digital’ amplification ............................................................................... 152

4.9 Class summary ............................................................................... 153 4.10 Introducing modes of control ......................................................... 155 4.10.1 Negative feedback modes .......................................................................... 156 4.10.2 Other Error Correction Modes ................................................................... 162

4.11 Conclusions .................................................................................... 164 References and Further reading ................................................................. 168

5

Features of the power stage 5.1

169

Overview ........................................................................................ 169

5.1.1 Operating with high voltages ..................................................................... 169 5.1.2 Operating with high currents ..................................................................... 170

5.2

Power devices ................................................................................. 171

5.2.1 Bipolar Junction Transistors (BJT) ............................................................ 171 5.2.2 MOSFETs (enhancement-mode power FETs) ........................................... 177 5.2.3 Insulated Gate Bipolar Transistors (IGBT) ............................................... 183

5.3

Recognising large signals ............................................................... 184

5.3.1 The slew limit ............................................................................................. 185

5.4

RF stability ..................................................................................... 191

5.4.1 Power stage, critical layout requirements .................................................. 191 5.4.2 Critical nodes ............................................................................................. 192

5.5

V&I limits on output, the context .................................................. 193

5.5.1 5.5.2 5.5.3 5.5.4

V-I output capability .................................................................................. 198 V-I output limiting (adverse load protection) ............................................ 200 Mapping V-I capability .............................................................................. 210 Audio protection, by fuse ............................................................................ 211

5.6

Clip indication – external relations ................................................ 213

5.6.1 Overdrive behaviour – internal relations ................................................... 215 5.6.2 Output stability and the output network (OPN) ........................................ 215 5.6.3 RF protection .............................................................................................. 217

5.7

DC offset, at output ........................................................................ 218

5.7.1 DC (Fault) protection (DCP, DCFP) .......................................................... 219

5.8

The output interface ........................................................................ 220

5.8.1 Muting systems .......................................................................................... 221

5.9

Output stage, cooling requirements ................................................ 222

5.9.1 Heat exchange ............................................................................................ 224 5.9.2 Thermal protection ..................................................................................... 226

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5.10 Logical systems .............................................................................. 228 5.11 Output transformers ........................................................................ 228 References and Further reading ...................................................... 230

6

The power supply

231

6.1

Mains frequency (50/60Hz) supplies ............................................. 231

6.1.1 6.1.2 6.1.3 6.1.4

50/60Hz EMI considerations ..................................................................... Surge handling ........................................................................................... Actively-adaptive 50/60Hz PSU ................................................................ Regulated 50/60Hz ‘passive’ supplies .......................................................

6.2

Supply amongst channels ............................................................... 240

235 237 238 239

6.2.1 Bridge benefits ........................................................................................... 242 6.2.2 Operation with 3 Phase AC ....................................................................... 242

6.3

Pulse-width power (PWM PSU) .................................................... 243

6.3.1 6.3.2 6.3.3 6.3.4

HF power supplies (SMPS, HF switchers) ................................................ Resonant power .......................................................................................... The higher adaptive PSUs .......................................................................... HF switching summary ..............................................................................

6.4

Power supply (PSU) efficiency round-up ...................................... 251

244 247 250 250

6.4.1 Amplifier efficiency summary ................................................................... 252

6.5

Power supply fusing ....................................................................... 253 References and Further reading ................................................................. 256

7

Specifications and testing 7.1

257

Why specifications? ....................................................................... 257

7.1.1 Types of spec. ............................................................................................. 257 7.1.2 Standards for audio power amps ................................................................ 259

7.2

Why test things ? ............................................................................ 259

7.2.1 Test tools and orientation ........................................................................... 260 7.2.2 Realtime test signals .................................................................................. 261 7.2.3 The test equipment revolution ................................................................... 264

7.3

Physical environment ..................................................................... 265

7.3.1 Mains measurement and conditioning ....................................................... 265 7.3.2 Power amplifier preconditioning ............................................................... 266 7.3.3 The test load ............................................................................................... 268

7.4

Frequency response (Bandwidth, BW) .......................................... 272

7.4.1 7.4.2 7.4.3 7.4.4

Gain and balance ........................................................................................ Output impedance (Zo) ............................................................................... Damping factor .......................................................................................... Phase response ...........................................................................................

7.5

Introducing noise ............................................................................ 281

7.5.1 7.5.2 7.5.3 7.5.4

Noise spectra .............................................................................................. Breakthrough and crosstalk (channel separation) ...................................... Understanding CMR measurements .......................................................... Measuring CMR .........................................................................................

7.6 7.7

Input impedance (Zin) ..................................................................... 286 Introducing harmonic distortion ..................................................... 286

274 276 278 278 282 283 284 285

7.7.1 Harmonics: the musical context ................................................................. 289 7.7.2 Harmonic distortion (THD, %THD+N) .................................................... 293 7.7.3 Individual harmonic analysis (IHA) .......................................................... 297

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7.7.4 7.7.5 7.7.6 7.7.7

7.8

8

Intermodulation (% IMD, Intermod) ......................................................... Dynamic intermodulation (% DIM 30/100) .............................................. Sundry intermodulation checks ................................................................. Other distortion tests ..................................................................................

298 302 303 303

Power output (Po)............................................................................ 304

7.8.1 7.8.2 7.8.3 7.8.4

Output voltage capability (Vo rms, MOL) ................................................. Dynamic output capability ......................................................................... Clipping symmetry ..................................................................................... Dynamic range ...........................................................................................

7.9

Dynamic tests ................................................................................. 308

7.9.1 7.9.2 7.9.3 7.9.4

Rise time (small signal attack) ................................................................... Slew limit (slew rate, large signal attack) .................................................. Transient response (impulse response) ...................................................... Peak output current capability ................................................................... References and Further reading .................................................................

Real world testing – rationale and procedures 8.1 8.2 8.3

305 306 307 308

308 309 310 312 314

315

Scope and why essential ................................................................. 315 Listening ......................................................................................... 315 Operable mains range ..................................................................... 317

8.3.1 Inrush current ............................................................................................. 318 8.3.2 Soft start ..................................................................................................... 319 8.3.3 Mains current draw .................................................................................... 320

8.4

Signal present indication and metering .......................................... 321

8.4.1 Clip indication ............................................................................................ 322

8.5

DC at the input ................................................................................ 322

8.5.1 RF at input .................................................................................................. 322 8.5.2 Large signals at input ................................................................................. 323

8.6

Output DC offset (output offset, Voos) ........................................... 323

8.6.1 8.6.2 8.6.3 8.6.4 8.6.5 8.6.6

RF at output ................................................................................................ Adverse loads ............................................................................................. Adverse load proving ................................................................................. Adverse loads, low loads and shorting ...................................................... Adverse loads, reactive .............................................................................. Hard drive testing .......................................................................................

324 325 325 325 327 327

8.7 8.8

Thermal protection and monitoring ............................................... 327 Muting behaviour ........................................................................... 328

8.8.1 Acoustic noise ............................................................................................ 328

8.9

EMI and EMC ................................................................................ 329 References and Further reading ................................................................. 330

9

Choice, application installation and set-up 9.1

331

Manufactured goods, a résumé ...................................................... 331

9.1.1 Choosing the right power amp, domestic .................................................. 332 9.1.2 Choosing the right power amp, for pro users ............................................ 335

9.2 9.3

Howlers ........................................................................................... 340 AC mains voltage ........................................................................... 342

9.3.1 Safety earthing ........................................................................................... 344 9.3.2 Mains cabling ............................................................................................. 346 9.3.3 Power factor correction .............................................................................. 348

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9.3.4 9.3.5 9.3.6 9.3.7

Mains fuses and breakers ........................................................................... AC mains connectors, amplifier end ......................................................... AC mains connectors, the power-end ........................................................ Mains wiring practice, domestic and studio ..............................................

349 350 352 352

9.4

Input connections............................................................................ 356

9.4.1 Balanced polarity and shielding ................................................................. 359 9.4.2 Quasi balanced (unbal-to-bal) .................................................................... 360 9.4.3 Input cabling ............................................................................................... 361

9.5

Output connections ......................................................................... 362

9.5.1 Speaker cabling .......................................................................................... 367 9.5.2 Impedance setting ...................................................................................... 370 9.5.3 Output polarity ........................................................................................... 370

9.6

Placement and fitment .................................................................... 371

9.6.1 Cooling airflow conventions ...................................................................... 371 9.6.2 Cooling and air pollution ........................................................................... 373

9.7

The 1 to 5 of prudent amplifier use ................................................ 374

9.7.1 System back-end troubleshooting .............................................................. 374 References and Further reading ................................................................. 376

10

Maintenance and surgery 10.1 10.2 10.3 10.4

377

Classifying failures ......................................................................... 377 Problem solving procedures ........................................................... 379 Universal repair procedures ........................................................... 379 Repair tools and equipment ............................................................ 389

10.4.1 Useful tools ................................................................................................ 389 10.4.2 Test powering tools .................................................................................... 390 10.4.3 Audio test tools .......................................................................................... 392

10.5 Testing components ........................................................................ 393 10.5.1 Testing BJTs (bipolar transistors) .............................................................. 393 10.5.2 Testing MOSFETs ...................................................................................... 395 10.5.3 Testing diodes, zeners and LEDs ............................................................... 397

10.6 Scope traces .................................................................................... 398 References and Further reading ................................................................. 400 Useful addresses for maintenance .............................................................. 400

Appendices 1 2

History ............................................................................................ 401 Makers’ listings .............................................................................. 409 A-Z, under principal use ............................................................................ 409 A-Z of integrated loudspeaker-amplifier makers ...................................... 413 Principal output device, by number of makers .......................................... 414

3

Active devices ................................................................................ 415 A Bipolar transistors, silicon ..................................................................... B Lateral power MOSFETs ....................................................................... C Power D–MOSFETs ............................................................................... D Thermionic valves (electron tubes) ........................................................

4

Index

x

416 420 421 422

Power amplifier terminology ......................................................... 423

457

Preface There has never been a book like this one, in its interleaving of electronics and audio, engineering ideality, and musical and practical reality. There haven’t actually been many books dedicated to audio amplifiers, period. On any level. Beginning with the electronics, amplifiers for driving loudspeakers are actually rather hard. So hard, that after 75 years, there is not a lot of convergence – not compared to say washing machines, which are similarly old. In spite of a century of consumerism, Music remains on a higher, primal level that interfaces with levels of human perception that precede and can outstep the logical. There have been many gifted minds at work in amplifierland, but they haven’t had even half the answers. Many have come unstuck, or lost the plot completely, confused by mathematical catastrophes in audio’s higher dimensions. Audio power amplifiers are unsung key tools in the immense growth of human massattuned consciousness during the 20th century. Imagine amplifiers were dis-invented. Without speaker-driving-devices, there wouldn’t be hi-fi systems, radios or (wild applause) TVs. There wouldn’t be PA systems, and there wouldn’t be any festivals bigger than village garden fetes, or at best 2000 seater amphitheatres. There wouldn’t be cinemas and recording studios, and no solid-bodied or electronic instruments. The huge emotional and psychic amplification, through music (its own capabilities also vastly expanded by electronic amplification, recording and processing) and the sound-tracked cinema (and video offspring) and their mass broadcast and affordability, that has made the 20th century vibrant like none before it, would be naught. The human world without good amplifiers – or any audio amplifiers – would be far less linked to spiritual and emotional heights – and probably not a lot quieter. ‘High Performance’ means that the book does not cover equipment where makers knowingly make significant ‘corner cuttings’ that degrade audio quality, reliability and utility, particularly so called ‘consumer’ and much so called ‘M.I’ equipment. This seems a more natural dividing line than any of the more common ones, like pro.vs.domestic. Everyone who is serious about music, wants much the same things, however much it needs adapting to suit their particular environment. What ‘High Performance’ does not mean is any particular price or other label of exclusivity. The amplifiers covered in this book could cost (at 1996 prices) £135 or £13,500. They could deliver 25w or 2500w, be used in the home, in the studio, a stadium, or in a field, so long as their aim or suitability is to permit the faithful reproduction of some kind of music and all its nuance. Across this book, you will discover that the contents’ focus purposefully veers from a wide pan across the most global, broad-minded considerations including amplifiers employing valves (tubes) and/or ‘zero’-feedback, wherein generalities are enough, through to a narrow concentration on the majority of modern transistor (‘solid state’)

xi

power amplifiers with global NFB, where the detail is magnified. Consideration of esoteric types has been exponentially tapered to avoid the book expanding to infinite volumes, while dove-tailing with the burgeoning number of new and reprinted books about alternative valve amplifier technology. More than any other you will see, this book fills-in and connects-up 101 missing details about audio power amplifiers.

Ben Duncan, Oxbows, Co. of Lincoln, England January 1996

To Amy and Jake, and to the many gifted musicians, singers and producers who have inspired my work

xii

Acknowledgments I am indebted to the following makers, designers, theoreticians, teachers, mathematicians, technologists, sound engineers, musicians, friends and colleagues – many of them ‘masters of sound’ – for their input either during the production of this book, or at some earlier juncture; and where asterisked (*), for their kind assistance in proof-reading, criticising or enhancing particular sections. They should not however be held liable for any errors or omissions! Ξ indicates valued assistance with the supply of pictures. In the UK: * Andy Salmon, MS&L. Bill Bartlett. Bill Huston, Aanvil Audio. * Bruce Hofer, Audio Precision. Ξ Charlie Soppelsa, Rauch Precision. Chris Hales, C-Audio. * Chris Marshman, YEC. Ξ Danny Cooklin, Turbosound. David Dykes. * Dave Newson. * David Cole, Turbosound. David Heaton, Audio Synthesis. Ξ David Neale, BSS Audio. Prof. Malcolm Omar Hawksford, University of Essex, Dept. of Electronics. * Duncan Werner, Music technology Course leader, & colleagues, University of Derby, Dept. of Electronics. Eddie Cooper, Audio Precision. * Gary Ashton, Fuzion. Graham Lust. Harry Day, Reddingwood Electronics. Heather Lane, AES . Ξ Ian McCarthy, MC2 Audio. Prof Jack Dinsdale Ξ Jerry Mead, Mead & Co. John Hurd. * John Newsham, Funktion One. * Lawrence Dickie. Dr. Keith Holland, ISVR.

Keith Persin, Profusion. Ken Dibble, Ken Dibble Associates. * Matt Dobson, Coastal Acoustics. * Mark Dodd, Celestion. * Martin Colloms, Colloms Electroacoustics. Martin Rushent. Michael Gerzon. * Neil Grant, Harris-Grant Associates. Norma Lewis, senior assistant, BDR. Norman Palmer, De Aston. Paul Freer, Lynden Audio. Paul Holden, ATMC. Paul Jarvis. Paul McCallum, Wembley Loudspeakers. Paul Reaney. Peter Baxandall. Peter Brotzman, Britannia Row Prods. Phil Newell. Phil Rimmer. * Richard Vivian, Turbosound. * Richard Dudley, B&W Loudspeakers. * Russ Andrews, RATA. Stan Gould, BSS Audio. Stephen Woolley, Fender Electronics. Ξ Steve Harris, Hi-Fi News. Steve Smith, Sound Department. * Terry Clarke, MC2 Audio. * Tim Isaac, ATC. * Toby Hunt, Funktion One Research. * Tony Andrews, Funktion One. Vince Hawtin, fanatic.

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In the USA: Adam Savitt-Maitland. Bill Steele, Spectrum Software. Bob Carver. Ξ Brian Gary Wachner, BGW. Dan Parks, NSC. Ed Dell, Audio Amateur Publs. Harvey Rosenberg, NYAL. Joe Buxton, Analog Devices. John Atkinson, Stereophile. John Szymanski.

Ξ Patrick Quilter, QSC. Roger Cox, Fender. Skip Taylor & Larry Hand, Peavey. Ξ Tim Chapman, Crest Audio. * Walt Jung, Analog Devices. Overseas Colin Park, ARX Systems, Australia. Conrad Eriksen, Norway. Tommy Jenving, Sweden.

Acknowledgment of other picture sources and production services: A.Foster & Sons, British Standards Institution, Canford Audio, Citronic, Crown Inc., Electronics World, Hi-Fi News & Record Review, Lincolnshire County Library Service, MAJ Electronics, National Physical Laboratory, Peter Gilyard-Beer, Pro Mon Co, SB, SoundTech, Stereophile magazine, Studio Sound & Broadcast Engineering, Spectrum Software.

Front cover Upper picture shows Brittania Row Productions' amplifier racks at work backstage during Pink Floyd's 1994 World Tour. Each contained four BSS Audio EPC-780, rated (per rack) at 10kW, with drive split between four frequency bands, to power Turbosound Floodlight and Flashlight horn-loaded speaker systems. Lower picture shows MC2 Audio model MC-650, which has microprocessor controlled auxiliary and protective functions, and it widely used in recording studio control rooms, as well as for PA systems.

Rear cover Pass Labs' Aleph 5 is a modern, high-end domestic amplifier, working in single-ended Class A, with an absolute minimum signal path, comprising only two MOSFET gain stages.

Method of Capitalisation The names of scientific units are capitalised broadly in accordance with the SI (Système International d’unités) convention, but there are exceptions when English Grammar, typographic values and visual communication take precedence. Units that are named after people are proper nouns and as such, should be capitalised: Amp(ère), dB (deci-Bell), Farad (Faraday), Henry, Hertz, Joule, Ohm, Volt (Volta), Watt. The SI convention (due to French origins) contrarily requires abbreviated forms to be capitalised. However, for typographic and visual purposes, the symbols ‘V’ and ‘W’ are usually set lower case when associated with numbers on their own: 3v, 100w, as the capitalised forms V or W are otherwise too dominant. Ampère is mostly spelt out in full – as well as being capitalised – in this book, to avoid confusion with amplifiers, since the two words – which will be encountered frequently – may both be abbreviated to amp and amps.

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System of presentation To write in depth and breadth about audio power amplifiers, many special terms are required, from disparate disciplines. Many of these are italicised when initially introduced, and most are translated or defined in the Glossary. If an unfamiliar word appears, look there. To ease reader’s immediate comprehension, occasional terms have been briefly clarified or explained parenthetically (in brackets). References appear at chapter ends, ideally numbered in the order in which they first appear in the chapter. ‘Further Reading’ then lists apposite literature that is not specifically cited, as it is either incidental or else so germane that it would be referred to throughout that chapter. Differences in technical terminology and practices outside of Britain, particularly those in the USA, are acknowledged (e.g. in brackets) where possible. Throughout this book, levels in dB referred to maximum output or clip reference point (r) will be cited as ‘–dBvr’ if voltage (v), also abbreviated dBr, where v is understood. Less often dBwr will be used if a power delivery level is being referred to full power (w) delivery. High performance audio power amplifiers is a long-winded description if repeated too often. Yet it alone is what is being focused on, a point not to be forgotten when the subject of the many sentences to follow is sometimes abridged, down to audio power amplifiers or just plain audio amplifiers or power amplifier, throughout this book. In places, familiarity with the capabilities of the PC (Personal Computer) is assumed throughout, as it is today the de facto workhorse in the world of all serious engineering, and much else. The creation and/or processing of nearly every squiggle of ink in this book was created with a trio of them Hercule, Hilary (after Sir Edmund) and Adelos.

References and Further Reading Books are distinguished from journals by having no months associated with the year, and if available, the ISBN is cited. Where journals span two months, the first month is cited. Where journals have floating publication dates, issues are referred to by nominal quarters, e.g. Q2 means 2nd issue in that year. Anonymous, faceless publications are placed under ‘Nameless’.

Abbreviations employed AN EPD EPR* ETI

Application Note Electronic Product Design (UK) Electronic Power Review (UK) Electronics Today International (UK, Austr.)

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EW, EW+WW Electronics World, formerly Electronics & Wireless World, formerly Wireless World, formerly The Marconigraph, (UK). HFN, HFN/RR Hi-Fi News & Record Review (UK). H&SR* Home & Studio Recording (UK). ISBN International Standard Book No. JAES Journal of the Audio Engineering Society. LCCCN Library of Congress Cataloguing card No. (USA) L&SI Lighting & Sound International (UK). Q Annual Quarter (1-4, or more). S&VC Sound & Video Contractor (USA). TAA The Audio Amateur (USA). WW Wireless World (UK). See EW, above. * Journals believed to be no longer published.

Publications Back issues of, or photocopies from, most journals cited, can be obtained by contacting the publishers. Journals Audio, 1633 Broadway, NY 10019, USA. Audio Engineering Society, AES, Room 2520, 60 East 42nd St, NY 10165 2520, USA. Or local branches worldwide. Electronic Industries Association, EIA, 2001 Eye St, N.W., Washington, DC 20006. Elektor - see TAA, below. Electronics Today International, ETI, Nexus House, Boundary Way, Hemel Hempstead, Herts, HP2 7ST, UK. Electronics World (EW, formerly WW), Quadrant, Sutton, Surrey, SM2 5AS, UK. Glass Audio - see TAA, below. Hi-Fi News & Record Review (HFN), Link House, Dingwall Ave, Croydon, Surrey, CR9 2TA, UK. Institute of Electrical Engineers (IEE), Savoy Place, London, WC2, UK. Institute of Acoustics (IOA), PO Box 320, St.Albans, Herts, AL1 1PZ, UK. Lighting & Sound International (LSI), 7, Highlight House, St.Leonards Rd, Eastbourne, East Sussex, BN21 3UH, UK. Sound & Video Contractor (S&VC), 9800 Metcalf, Overland Park, KS, 6621-22215, USA Speaker Builder - see TAA, below. Stereophile, 208 Delgado, Sante Fe, NM 87501, USA. Studio Sound & Broadcast Engineering, 8th Floor, Ludgate House, 245 Blackfriars Rd, London, SE1 9UR, UK. The Audio Amateur (TAA), PO Box 576, Peterborough, New Hampshire, NH 03458 0576, USA. (Same address for SB, GA, Elektor). Newsletters – concerning software used to create the graphs in this book: Audio Precision (test equipment), Audio Precision, PO Box 2209, Beaverton, Oregon, 970753070, USA. Spectrum News (MicroCAP simulation software), 1021 South Wolfe Road, Sunnyvale, CA 94086, USA.

xvi

1 Introduction and fundamentals

“Why do rhythms and melodies, which are composed of sound, resemble the feelings; while this is not the case for tastes, colours or smells ?” Aristotle

1.0. What are audio power amplifiers for ? In sound systems, power amplifiers are the bridge between the loudspeakers and the rest of any sound system. In everyday parlance, ‘Audio Power Amplifier’ gets abbreviated to ‘amplifier’ or ‘amp’. But all audio power amplifiers (other than those that drive vinyl disc cutter-heads) are really ‘loudspeaker drivers’. The definition is global if earpieces and headphones are included. Sometimes, amps are combined with the speakers, forming ‘powered speakers’; or they may be packaged with the preceding equipment functions, e.g. as in domestic ‘integrated’ hi-fi amplifier + preamplifier, or a band’s ‘mixer-amp’.

1.1 What is the problem ? A sometime bass player, and foremost international writer on, and reviewer of, audio quality explains “If you read electrical engineering textbooks, you’re left with the impression that the audio amplifier is a well-understood, lowly sort of beast, compared with radio-frequency circuits. ... All I want is an amplifier that performs its simple task in an accurate, musically honest manner. I can’t think, however, of an amplifier which can do this without crapping out at high levels, or obscuring lowlevel detail, or flattening the soundstage ... , or changing the (tonal) balance of the speaker ... , or adding metallic sheen, or loosing control of the speaker’s bass so it booms, or gripping it so tight that music looses its natural bloom, or doing something - whatever it is - that destroys the music’s sense of pace.” [1]

Introduction and fundamentals

1.2 What is audio ? Throughout this book, audio, music or programme (program in US) are all shorthand for ‘music reproduction, or production or live amplification’. Other than the sounds of acoustic instruments and the outputs of electronic ones, music programme may include speech, gasps, birdsong, street sounds, sonar beeps, toad clucks, or any other sounds whatsoever that are employed for musical purposes. ‘Sound’ is synonym for audio in modern usage, at least in the context of sound system’ and ‘sound reproduction’.

1.3 What’s special about audio ? The amplifiers in this book are about reproducing music but they are equally applicable to the amplification or reproduction of speech, where the highest qualities and nuances of the living voice are of importance. These include religious and spiritual ceremonies, plays, poetry and chant. What’s special about all of these – all really variants of music – is that they involve sounds that make direct contact with powerful, pre-verbal centres of the human mind, affecting conscious states, ultimately in the higher direction of ecstasy [2] [3]. Music is not just ‘the art that the other arts aspire to’, but it is in there with the highest, most transcendent mind events that can be experienced by human beings. The human eye and ear are both amazing for the range of levels, or dynamic range, over which they can resolve differences and operate without damage. The range in both cases is up to at least 1000 million times (160dB). The eye can perceive a single photon. The ear can perceive the result of air moving over a distance equal to the radius of a hydrogen atom, the smallest building block of all matter in the cosmos. Of the two, listening is humankind’s most wideband sense, spanning 10 octaves. Whereas everything we see is compressed into just one octave of light! The reproduction of music is a multi-dimensional event. Music involves instantaneous changes in Sound Pressure Level (SPL). The listeners’ instantaneous perception of musical values depends on what has gone before. The changes are driven by two things that have no physical reality – the way music is structured in time and in pitch. As these two are not causally related, and the sound already has a ‘where’ (humankind’s everyday 3D co-ordinates), at least 5 dimensions must be involved [4,5,6]. As we live in lower, four-dimensional space-time (3D = 3 dimensions of space, and 1T = one dimension of time), the human brain is not able to ‘see’ the whole picture, only segments at a time. This broadly explains why the traditional scientific method has been so badly dented by its attempts to prescribe the optimum means of music reproduction.

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1.5 Some different aims of sound reproduction Some two hundred years ago, William Blake, the English visionary polymath wrote: “For man has closed himself up, ‘till he sees all things thro’ the chinks of his cavern.” William Blake, 1757 to 1827 Another reason, is our increasing mis-apprehension of quality. This symptom of 20th century living is considered again in chapter 9.

1.4 The ramifications of quality on audio It is important for all sound system users to be aware that music’s subtler qualities and intended communication may be restricted or even prevented when an amplifier damages or twists the signal which represents (is an analog of) the music. To ‘damage’ and to ‘twist’ are forms of distortion. There are many names for the different ways in which this can happen. Some ways are blatant, others subtle. When music is distorted, it not only looses it subtler essence; it can also hurt physically. Undistorted music, even at extremely high peak SPLs, as high as 140dBCSPL, is not painful to engaged listeners and will not immediately harm healthy ears. The majority of hearing damage is mainly caused by, or greatly exacerbated by, industrial and urban noise [7]. Explosive and percussive sounds can have instantaneous levels that are 20dB (10x) above what ordinary SPL meters and acoustic spectrum analysers capture. The likelihood of any short or long-term hearing impairment is greatly exacerbated by distorted sound systems. Inadequately rated or designed power amplifiers are just one contributor to this.

1.5 Some different aims of sound reproduction “Experience which is not valued is not experienced .... Value is at the very front of the empirical procession” Robert M. Pirsig, Lila [8] Historically, since the birth of sound recording in the late 19th century, the idealist aim of quality recording, and the quest of Hi-Fi (‘High Fidelity’) sound reproduction equipment has been to capture, then reproduce at any later time, the captured sound with as much accuracy as possible. Intention thus defined, perfection has been attained only when the reproduction has more accuracy than the sharpest human perception [9] . This approach is still widely mis-named (as if narrow-mindedly) ‘concert hall realism’. A better, more global (if clumsy) description of what is sought, is ‘full sonic capture of a music event’. Such ideals always beg the question ‘which seat ?’, since the ‘reality’ of all musical events depends on where the participant is. As in any other perfectionist ‘event recording’, the 4 dimensional manifold (w,ht,d+t) of human perceivable reality is probed. There is no singularity here.

3

Introduction and fundamentals Some people prefer and justify the use of considerably inaccurate replay systems (including power amplifiers) on the grounds that source material is mostly very considerably inaccurate. Unable to perceive pleasure as something beyond time and linear measure, they cannot see that spending £1000’s to get immense pleasure from music that might total just 1% of a collection, or just 680 minutes (say) could possibly be justified. The closely guarded secret is that ecstatic states are timeless. Others may naively hope to, and the fine artisans do achieve, some kind of degree of cancellation of the inaccuracies (e.g. partnering a ‘slow’, dull amplifier with a ‘fast’, speaker with ‘brittle’ treble). Others, loving alcohol and rich food too much, perhaps, bask in the creation (often with valve equipment ) of a euphonic, edgefree or ethereal sound that never existed in the recording session. Or their stance may simply reflect that for them, music is a second-division interest, into which they cannot afford to invest any further. Since the mid 60s, along with rock’n’roll music (in its diverse forms), an alternative, openly hedonistic definition of what some recording producers and users of sound reproducing equipment are seeking, has developed: that music is sought and created by humans to generate or aggregate ecstatic and blissed states, and the purpose of sound reproduction is to make such higher states more available [2,3].

1.6 About people and their hearing A significant number of people (possibly 0.1% of the population – which makes several million worldwide) have unexpectedly sensitive hearing. Compared to the average figures reeled off in acoustics and electronics text books, the perceptive ability of some individuals with music and chant, (not necessarily speech) extends up to ten times further out in audible frequency, pitch, harmonic content, signal delay, level or phase – amongst others. One example is that some people are as sensitive to 4Hz, a frequency often described as ‘subsonic’ (inaudible except through bone conduction), or strictly ‘infrasonic’, at one fifth of the frequency where ear hearing in most people ceases. Another is pitch discrimination. It has been noticed that some musically trained listeners can detect the difference between tones only 0.1% apart, up to at least 10kHz. This implies the human ear + brain combination is sometimes capable of resolving timing differences of around 100nS or a tenth of a millionth of a second. Such abilities are natural in some people, and learnt in others. Differences that are identifiable to some of these ‘golden eared’ listeners (and that can even cause involuntary physical reactions such as a feeling of sea-sickness) can correlate with differences in basic, conventional audio measurements of only 1 part in 100,000 or even less. Other differences may be just 1/30th of the immediately preceding signal, yet it has taken 10 or 20 years of discussion before they are measured [10, 11]; commonplace, simplistic measurement techniques with unrealistic test signals cannot ‘see’ them at all.

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1.7 Limits of a ‘objectivity’. Why listen ? Sensitive listeners, once they have self-confidence, will notice differences in music recordings they love and know well, when sound equipment is changed. After dismissing well established, simple reasons for sonic differences (such as a slight level mismatch causing the louder unit to sound brighter, a facet of loudness perception), the fact remains that what is judged to be sonically better nearly always reveals details and ambiences that were not previously audible. Moreover, nearly every audio amplifier is perceived by skilled listeners to have a sound signature of its own. Yet such basic technology as Hi-Fi is (presumably) assumed by the general public to have reached 99% of what is possible. Instead, the experience of the high-end should be warning us that what the public hear is probably barely 5 to 10% of what is possible, and the best systems are still pushing at the 50% barrier and all differ in their particular ‘bestness’. An amplifier – as our servant, and the speaker’s master, is not supposed to add or subtract from the performance. One of the worlds’ grandmasters of high-end amplifier design, Nelson Pass, reminds us that, past a point, there is no ‘best’ amplifier “... just as there is no best painting or best wine” .

1.7 Limits of ‘objectivity’. Why listen ? The traditional ‘brute force’ approach to overcoming individual sound signatures is to make conventional performance-indicator measurements very good so differences don’t matter, which presumes that what is being measured says all about what is heard. Loudly written specifications claiming ‘zero distortion’ or ‘ultra-low distortion’ has over the years tricked many millions of users into thinking the sound had to be good. This approach, known as ‘hearing with the eyes’ is still taken to extremes in obsessively technical, ‘hard line objectivist’ circles. Much conventional measurement is like claiming a house to be utterly perfect because its sides are at 90.0000°, i.e. it has highly accurate orthogonality – while other relevant aspects that are not being measured (in this case, say the audio equivalent of a house’s damp-proofing) may be catastrophically bad. Many measurements are made because they are easy to make because test equipment exists for them, and because such equipment is commonplace, easy to use and easy to buy. But the original relevance of the tests has mostly been forgotten and is rarely questioned. Even the more modern and sophisticated measurements are made to look small by such a complex signal as music. As a measure of this, there are still no real-time, errorcomputing spectrum analysers able to span audio’s 0-200kHz with a 160dB dynamic range. British loudspeaker designer Mark Dodd sums the situation up in the equation: (Music + recording + playback electronics + ear + brain) = complex problem Since the mid 70s, amplifier users who are confident in their aural judgment have increasingly learnt to trust their ears with little or no recourse to specifications and measurements, except to check and pass them for basic standards. Outside of electronics and the purely technical, the subjective approach to decision making is the norm on every level.

5

Introduction and fundamentals An amplifier is not supposed to add or subtract from the performance. Yet if conventional measurements are all, then the audibility of some variations of the parts in amplifiers indicates that some human brain-ears are reacting to (as mentioned earlier) differences well below 1% (1 part in 100) and in some cases, 1 part in 100,000 or even less. To make sense of this without invoking the 4 spacial dimension, consider the ink in a book. The ink makes the 500 pages into 1 million words of knowledge. But it only makes the book weigh one gram heavier. If the book can only be analysed by weighing on scales, then the quality difference between it and another same-sized 500 page book on a completely different topic will be hard to measure. Humans who can ‘read’ may be detecting differences in ink weight of 1 part in 10 million. The order is implicate! [12]. Overall, to be suitable for its intended purpose, even the cheapest audio equipment has to designed with a more global and detailed attention to engineering detail, than any equivalently power-rated ‘industrial’ amplifier.

1.8 Why are power amplifiers needed for audio ? For the most part, the processing of audio signals can be performed with only minuscule power, either input or dissipated. Analog signals pass through the majority of the overall signal path at average levels in the order of 100mV to 1 volt. Load impedances may be as high as 100kΩ but even if as low as 5kΩ, only 120µW (a hundred and twenty microwatts; or about a tenth of a thousandth of one watt) would be dissipated. At this rate, it would take about eight million hours or hundreds of years of playing, for the load to absorb or use one unit (1kWh) of electricity ! Most loudspeakers used to reproduce audio are highly inefficient. Typical efficiencies of common direct radiating speakers are 1% to 0.05%. By comparison, the efficiency of an internal combustion engine (considered highly inefficient by ecologists) is between 2500% and 50,000% greater. A medium sized car uses about 70kW to move 4 people or hundreds of pounds of goods, and its own weight – altogether at least half a tonne, at speeds of say 70mph. In some sound systems, to move just the weight of air molecules to reproduce a bass drum, as much as 7kW of electrical ‘fuel’ can be burned in bursts. And yet a loudspeaker only needs to convey 1 acoustic watt to the air to recreate music at the highest practical sound levels in a domestic space, i.e. about 120dBSPL. And a tenth of this level (0.1 acoustic watts) will still suit most of the loudest passages in the less extreme forms of music. If speaker efficiency is taken as 0.1%, and 1/10th of an acoustic watt is enough, then an electrical input power of 1000 times this is needed, i.e. 100 watts. The highest SPLs in music can be considerably greater than 0.1 acoustic watt. Loudspeaker drive units exist that can handle short term electrical power bursts (the norm in much music) of 5000 watts (5kW) or more. With 2% efficiency, today’s most capable drivers can generate 100 acoustic watts each. With horn loading,

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1.8 Why are Power Amplifiers needed for audio ? efficiency can be raised to 10% or more, allowing one drive unit to produce 500 acoustic watts for large scale PA. This allows fewer sound sources to be used, improving quality. When comparing SPL figures it is helpful to remember that at medium SPLs (sound levels) and mid frequencies, a tenfold increase in watts offers only an approximate doubling in loudness to the ear. But at the lower bass frequencies and at higher SPLs, considerably smaller changes in wattage, say just x3 to x5, have the same doubling effect. SOUND PRESSURE LEVEL dB re 0.0002 MICROBAR

LOUDNESS LEVEL (PHON) 120

120

110 100

100

90 80

80

70 60

60

50 40

40

30 20

20

10 MAF

0 20

100

1000

5000 10,000

FREQUENCY IN HERTZ Figure 1.1 Robinson & Dadson free-field equal-loudness contours, showing how average human hearing sensitivity to pure tones, varies with frequency and level, when facing the sound. Made at the UK’s National Physical Laboratory in the 1950s, these are superior in accuracy to the older, more famous curves, made in the 1930s by Fletcher & Munson, at Bell Labs, USA. But they are still only approximate for music and for individual ears. On the left is the SPL (sound level) at your ears. The levels on each curve are in Phon, a level unit which like each curve, follows the average ear’s sensitivity. Notice how the 120dB dynamic range of the midrange is squeezed down to about 60dB (a 1000-fold ‘space’ reduction) at a low bass frequency (20Hz). And how sensitivity around 3.5kHz because increasingly acute at high levels. ‘MAF’ is the average threshold of perception in a very quiet space. But it is not the end of sound; some people and many animals can hear 20 or more dB below the MAF level. Courtesy National Physical Laboratory

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Introduction and fundamentals

1.9 Music fundamentals Music has a number of key qualities. In the beginning, there are (amongst other things) particular tones. Some tones (fundamentals) are harmonically (numerically) related to others having higher frequency. These belong to a tonal subset called harmonics. Together, fundamentals and harmonics, and their phase relations, along with the envelope (the ‘shape’ of the sound developed by the averaged amplitude) create a timbre. Tones which are not harmonically related to anything may be discordant. If they are harmonically related, but adversely (usually odd harmonics above the 5th) they are dissonant. When a tone changes in intensity, its ‘frequency’ (as perceived by the ear) changes. Pitch is (crudely) the musician’s ears’ own version of frequency and level co-ordinates. Sounds (often from percussion) that have no dominant, identifiable tones are atonal. They are akin to noise bursts. Continuous sound, both tonal and atonal, get boring after a while. The tonal waveform changes over a short period (its period is usually measured in milliseconds) but each subsequent cycle is identical to the first. Continuous atonal sound is considerably more interesting, or relaxing as a waterfall can be, say. Music’s higher vital component, its dynamic, differentiates time. Tonal and atonal sounds fade and increase, stop and start, and change in pitch and frequency in diverse patterns, creating wave-patterns (as seen on an oscilloscope) that rarely repeat exactly, and would appear madly chaotic to an alien creature without ears. The overall amplitude (size, loudness) pattern is called the envelope. 2.5 Input data – volts 2.0 1.5 1.0 0.5 0.0 -0.5 -1.0 -1.5 -2.0 -2.5 auto 0.0 1000.0 2000.0 Time – msec Figure 1.2

3000.0

4000.0

In this ‘amplitude-time domain’ recording off a Fender Precision bass guitar’s E-string, the waveform (read from left to right) is seen decaying over 4000mS (milliseconds) or 4 seconds. The envelope is the outside shape. Hence a ‘decaying envelope’. The darker parts are where the waveform density is greatest, due to higher frequency inflexions. Notice the initial transient spike, up the top left side, is positive going. Otherwise the envelope is asymmetric - as the signal has a higher negative amplitude, at least for the first two seconds. Reproduced from Stereophile magazine, with permission

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1.10 Adjectives that describe sound Timing retains the music’s meaning, concerning the precise schedule of the beginning (attack) and build up of each tonal and atonal building block, and its sustain (levelling off), decay and release. 5.0 4.0 3.0 2.0 1.0 0.0 -1.0 -2.0 -3.0 -4.0 -5.0 auto 290.0 Figure 1.3

300.0 Time – msec

310.0

320.0

Fender Precision bass guitar’s E-string, with the amplitude-time window magnified, out to 40mS (0.04 seconds). Shows overall asymmetry. The transient is the positive dwell point (at 288ms), followed by negative spiking (at about 289 mS). Reproduced from Stereophile magazine, with permission.

In most recording locations, sound is reflected off nearby surfaces, causing multiple early reflections or ‘reverb’(eration). The added complexity is heard as richness. See ‘Dynamics’, above, and on page 12. Complex distortions, both gross and subtle, caused by mics, speakers, electronics and cables can cause deviations in what the musically adept and experienced ear expects. Tonal qualities can be unduly emphasised or retracted, timing thrown out of sync, subtle dynamic contrasts and ‘edges’ blurred, and spacial qualities bizarrely warped or flattened.

1.10 Adjectives that describe sound Despite the fact that music drives a large part of all human art throughout history, and predates all technology, and despite the fact that everyday English speaking calls upon tens of thousands of words, the right words are oddly sparse when it comes to describing how ‘sound’ sounds. Even the long established technical vocabulary of music composition is small in comparison to other fields. In a technically educated person’s vocabulary of some 10,000 to 50,000 words, it’s hard to find even 100 that are widely used for audio performance description. On the following pages are 96 terms, set in a musical context.

9

Introduction and fundamentals

1.10.1 Tonal qualities Adjectives which describe narrow tonal qualities or irregularities are listed in the following table, in order of frequency. The dividing lines between bass, mid and treble are always arbitrary. ‘ + ’ indicates the adjective is used to describe an excess in the frequency area. ‘++’ means ‘even more than’. ‘ – ’ indicates a deficit. No symbol means the word is not simply a tonal indicator, or is used mainly for frequency area identification. Words on cascaded lines are synonyms.

ñ Highest Treble Airy Sheen +Tizzy

see ‘Broader tonal perspectives’. Very high treble, above 16kHz, often absent or really hiss. Excess around 12-16kHz, usually overemphasizing cymbals’ high harmonics.

+Hard, +Metallic, +Brittle Excess of high, metallic-sound, usually around 8-16kHz. +Bright, +Brilliant, ++Glassy Excess around 4-8kHz. Sibilance 5 to 7kHz.

Treble, down into the High Midrange +Crisp Presence +Nasal +Honky +Chesty +Boxy +Barky +Woody

Peak about 3 to 4kHz. Centered on 2kHz. 1kHz emphasis. Like a Cockney saying ‘oi’, or like poor or improperly used mid/hf horn speakers. Around 600 to 800Hz. Excess in the 200 to 400Hz area, particularly with pure male vocals. As if the singer is inside a cardboard box. Aggravated by cube shaped monitors. Around 250-450Hz. A characteristic mid-bass resonance in some larger speakers.

Low Midrange into Bass Boomy Punchy +Balls, +Ballsy, +Gutsy Boof-Boof + Chunky – Gutless

see dynamics. Around 120–160Hz, a high definition area. Low bass that is visceral, i.e. can be felt. Around 80–90Hz, soft bass area. 80–90Hz ‘sample’ bass with added harmonic definition. Absence of low bass.

ò Lowest Bass 10

1.10 Adjectives that describe sound

1.10.2 Broader tonal descriptors Airy Closed in Dark Dull Enclosed Lean Muffled Open Recessed Rich Thin Tune playing

Smooth, apparently effortless high treble, seeming to extend further than the music. Suggests ultrasonic capability. Treble lacking above 10kHz, almost the opposite of airy. Sound that tilts down from the bass upwards. General lack of treble. Dull, coloured, airless. Slight, gentle reduction below 500Hz, or very clean, transparent bass. Where high frequencies are reducing rapidly, above 2kHz. See Airy. General lack of midrange A downtilt in level above 300Hz. Also, a slight excess of reverb. Overall lack of bass. A consistent quality (especially power) throughout the bass range. Opposite of ‘One note bass’.

1.10.3 General sonic adjectives and nouns Aggressiveness Preponderance of mid-high energy (3-6kHz), often phasey and distorted. Ambiance Not a sonic adjective, but may be portrayed in recordings. To do with mood and feeling. Elusive Neptune–Venus stuff. Ambience Usually subtle, low level non-musical background sounds captured in a recording, that are usually only subliminally appreciated, but add to the sense of the occasion. Analytical When sound equipment seems to reveal too much of the stitchwork in music. Sometimes used when a system has distortions that unduly emphasise detail or ‘edges’. Articulation When you can hear the inner detail of complex sounds, particularly those in the main vocal range (300Hz-3kHz). Clinical Suggests sound that is clean, bright, sharp, detailed, but may be mildly pejorative, as cleanliness in one area shows up dirt elsewhere. Also suggests emotional qualities are held back. Detail – see Space Dry Sound tending to lack reverberation. Euphonic Erring on the side of being pleasing at the expense of accuracy. Fuzzy A spiky yet soft texture caused by high distortion and compression. Glare Distorted mid treble. Also tonal imbalance or forwardness. Grainy Excess texture. A kind of distortion usually in the high midrange. Gritty Like grainy, but harsher and coarser. Grunge Like gritty, but more muffled. (Actual Grunge music is closer to being gritty) Hardness Fatiguing ‘wood block’ type of midrange emphasis. Harsh Dissonant and/or Discordant. Unpleasant.

11

Introduction and fundamentals Loose Lush One note bass Phasey

Transparent

Woolly

Badly damped bass See Rich under Dynamics. Poor damping of major resonance(s) in low bass. Symptomatic of a frequency response that undulates like a comb. Co-exists with a manic, zig-zag phase response, literally ‘phasing’ our hearing system. When you feel you’re hearing just the music, not the replay equipment. A sense of ‘nothing in the way’. Being able to hear back to the recording venue. In Martin Collom’s words “Vital aims !” - see Loose.

1.10.4 Dynamics The next group deals with how accurately the music at many levels and frequencies is output over time. The comparison can be loosely likened to lowering a small plaster model of a huge mountain range over a copy, to check the fit. Representing 50 miles of complex 3D surface (Time, Energy and Frequency), it has fine detailing to the nearest inch of real mountainside, symbolic of the ear’s ability to resolve millionth-sized differences buried deep in the main mass of sound. Boomy Poor bass damping. A bad loudspeaker/amplifier combination. Congested See Smeared and Thickened, which are facets of the same effect. Dynamic contrast Subtle changes in level or pitch embedded amongst much larger changes. Dynamic range In audio engineering, the amplitude performance envelope of a sound system. In music parlance, the programme’s intensity range. Audiophiles call this ‘Dynamic contrast’ to distinguish. Fast Incisiveness of attack, particularly of bass fundamentals, but as bass doesn’t ‘move fast’ by definition, most likely a reflection of rapid damping, proper synchronisation between the fundamental and harmonics (see Chapter 7), and correct reproduction of all associated harmonics. Incisive Conveying the ‘slicing’ sound of close miked snares, like a sonic machete knife. Indicative of good attack synchronisation, like ‘Fast’ and ‘Slam’. Lifeless Superficially perfect, anodyne reproduction conveying nil emotion or interest. Commonly caused by forcing equipment or system to manifest a perfect measured frequency response without regard for factors affecting space or dynamics. Micro-dynamics Lifelike energy (transients) in small, low-level sounds. Muddy Especially applied to bass; see ‘Smeared’. Pace Ability to make music seem to unravel at the pace (or BPM) it was recorded at, rather than slower. See ‘Fast’. Punchy Similar to Slam, but can have a pejorative element of ‘One note bass’.

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1.10 Adjectives that describe sound Rich(ness) Rounded Rhythm Slam Slow Smeared Solid Squashed Thickened

Transient

Lots of coherent reverb. More usually applied to program rather than equipment. Loss of attack transients, due to poor damping, poor hf response, or slew limiting. Ability to put across the infectious ‘vibe’ inherent in a live show, that makes people want to dance or move in rhythm. Convincing, correctly synchronised attack for a fundamental in the 125Hz area. Rhythm seems slower. Caused by excess incoherent reverb, too much harmonic and intermodulation distortion and/or timing errors. Well damped bass. May be caused by hard limiting. Seeming absence of most dynamic contrasts. Can be caused by compression or soft limiting, or more subtly by any path component, from mics to resistors. Reduced dynamic contrasts. Abrupt, short lived events in music. Skilled ears can resolve differences in attack slopes and harmonic synchronisation down to tens of microseconds.

1.10.5 Space Dimensional qualities are embedded in recorded music. Even a mono soundfield can yield spacial information. Good stereo can create 3D images, but as a low grade hologram, you can only experience it from one side, not be in it. It is nonetheless capable of offering a deep experience, and is still revealing its holographic recovery capabilities, over 60 years after its simultaneous invention in the UK and USA. Users of higher dimensional encoding systems (eg. Binaural, Ambisonics, Holophonics or equipment like the Soundfield Mic or the Azimuth Co-ordinator) can create full sonic holograms, ie. soundfields (or dynamic sculptures) you can walk around and get inside. Detail Contrast Etched Focus

More spacial version of Dynamic Also the diametric of Muffled. Finely detailed. Sharpness of detail. May vary across the soundfield, in all 3 dimensions. Image, Imaging Ability to portray width, depth, and sometimes height. Layering Sounds having a precise depth in a soundfield, with the implication of many depths or infinite gradation. Hologram When coherent and correctly focused light or sound enables higher dimensions to unfold. Pinpoint When the image is very stable and finely etched, like some metal sculpture.

13

Introduction and fundamentals Smeared Sound stage Timing

When an otherwise sharp image seems to be portrayed through butter-smeared glass. See also under Dynamics. Also Timing. The space between and around two speakers in which sound in stereo (Greek for ‘solid’) appears to emanate from. Time is a another kind of spacial dimension. The timing between sound components at different frequencies coming from one or more instruments may be unlike the original sound. Compare bass to mid, bass to treble, etc. Delays of milliseconds or less can be audible.

1.10.6 Botheration or Abomination When something sounds atrociously bad, e.g. “Like a box of rifles” An unbearably crashy, thuddy sound. “Like sandpaper:” An unnervingly scrapy, scratchy sound. “Like a train crash” A frighteningly loud metallic aggression attack. “Blanketed” A heavily dulled, possibly slow, tiring sound.

1.11 Nature and range of music (alias programme) (i) Asymmetry All sounds comprise alternate compressions and rarefactions of the air (or other physical medium), ie. pressure cycles back and forth. For a completely pure tone, the net change in air pressure is nil (assuming an average of whole numbers of cycles). For most musical sounds however, the waveshape that describes the pressure moment-by-moment is for periods ‘more in one direction than the other’, skewed or lopsided. This is called asymmetry. See Figures 1.2 and 1.3. In a well sealed room containing music, it would have the effect of very slightly varying the atmospheric pressure over periods of several seconds. When music programme is converted into an analogous electrical waveform, asymmetry causes DC voltage shifts, that can upset operation. This is most likely in amplifiers of certain topologies, and that have only been tested with conventional test signals, which are all perfectly symmetrical.

(ii) Transients All kinds of music in its raw, live state often has unpredictable bursts which can be 6, 10 or 20dB higher than the average pertaining up to now, or for some time. Fast edges, lasting less time than an SPL meter needs to respond, can be 20 to 40dB above the average short term levels. The ear barely registers these, but notices when they are missing, and notices for sure something nasty is happening when edges cause the signal chain to hiccup for a much longer period. In the recording process, and in PA systems, transients are ‘rescaled’ with compressor-limiters. These ‘squash’ rather than excise the effect. As a result, transients in common recordings are at most often only 15dB above the average level.

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1.12 Bass & Subsonic content The average difference between the average (or mean or rms) level, and the highest peak levels of programme, has a name. It is called P.M.R, alias Peak-to-Mean Ratio see section 2.4.4. This term subtly distinguishes it from Crest Factor, which has a similar definition but to be applied only to regular, uniform waveforms. PMR is initially determined by the genre of music. Some approximate examples are: Orchestral works 18 to 30dB Rock 9 to 30dB ‘Muzak’ 3 to 9dB

(iii) Variability Music is not predictable. There is no algorithm (as yet) and computers are still incapable of recognising signals which are music and those which are noise. Within the parameters 0, +145dBSPL, 10 – 100,000Hz and zero to n thousand seconds, almost anything can happen.

1.12 Bass and subsonic* content * acousticians prefer ‘infrasonic’

Bass (lf, LF, low frequency) energy in music is the content with frequencies below 300 Hz (Hertz). Taking 20Hz as the lowest limit, the region we characterise as bass encompasses half the octaves that music spans [13]. Not forgetting our logarithmic perception of frequency, this means that every Hertz difference in frequency counts for more, the lower the frequency [14]. The Robinson and Dadson equal loudness contours (Figure 1.1) are an essential starting point to understanding how sounds at bass frequencies differ from midrange sounds, and particularly how their useful dynamic range is compressed naturally by the ear. Below 150Hz, bass becomes increasingly visceral amd tactile. At the high (110dB+) SPLs at which heavy metal or hard rock is performed, kick-drum frequencies centered around 120Hz can be felt in the solar plexus. These are literally ‘hard’ and many listeners may find them offensive or at least unpleasant. Below 100Hz, bass softens. Reggae, Funk and House music make full use of the ‘pleasure’ region, centered on 80Hz. PA system SPLs in this range have many times been observed peaking at up to 135 to 145dB, i.e. 15 to 20dB beyond the supposed ‘pain threshold’ without harm or even physical discomfort. Indeed, many listeners describe the experience as cathartic, healing and giving rise to raised consciousness. These lowermost audible frequencies are also the ones most strongly experienced when hearing underwater – an environment like that in which all human life began. Below 40Hz, bass (from a more limited acoustic, but still infinite electronic repertoire of instruments) becomes more tactile again, and by 16Hz (or some lower frequency depending upon physiology), it is no longer audible through the ear, but solely through bone conduction. This alters the way we hear, since sound is transmitted much faster through solids (the earth, the floor, the feet, the skeleton) than through the air. It is therefore possible for sub-bass signals to be ‘felt’ ahead of the higher, audible components. This effect may be noticed in thunderstorms.

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Introduction and fundamentals In most music, there is little explicit, large signal ‘sub-sonic’ musical content. The lowest, large amplitude fundamentals are made by electronic means; or acoustically by pipe organs, giant gongs, and long horns. Other instruments may make subharmonics (eg. the 3rd sub-harmonic of 41Hz, the fundamental frequency of the lowest note (E) on a bass guitar, is approximately 14Hz), but only in small amounts. Power Spectrum Magnitude – dB volts -5.0 -15.0 -25.0 -35.0 -45.0 -55.0 -65.0 -75.0 -85.0 -95.0 -105.0 auto 10.0 Figure 1.4

100.0 log Frequency – Hz

1000.0

Fender Precision bass guitar’s E-string, showing amplitude-frequency, i.e. spectral content, during the initial decay period after the Figure 1.2 transient. Reproduced from Stereophile magazine, with permission.

At frequencies below the fundamentals of musical instruments, say below 15Hz, much sub-bass is present in the real world even if it is just the rumble of the stage boards, of underground trains and the air conditioning. If these sounds are wholly excluded from a recording, it can seem disconnected from its reality – at least on systems capable of reproducing low bass at high levels giving it audibility. Also, the abrupt filtering needed to remove subsonic content delays wanted, legitimate bass signals at slightly higher frequencies, with respect to the midrange.

1.13 HF dynamics and ultrasonic content High frequencies (hf, HF, treble) begin at around 5kHz. Higher still, peoples’ hearing rapidly becomes insensitive at their upper limit. Depending on your genetic makeup, diet, health, age, and cumulative exposure to non-musical percussive sounds (particularly hammering and gunshots), the upper conscious limit at quite high SPLs (100dB) typically varies between 12 and 20kHz. In spite of nearly 20 years exposure to high intensity music, up to 145dBSPL,the author’s hearing presently rolls-off at about 17.5kHz.

Energy Vs. Power Above 3 to 5kHz, the average power levels of nearly all kinds of music, integrated over a minute or longer, are less than the levels at lower frequencies, and reduce

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1.13 HF Dynamics and Ultrasonic content further with ascending frequency [15]. As a rule of thumb, the power in the mid to low bass regions is at least ten times (20dB) more than the power at 10kHz. A great body of music revolves around relatively abrupt change. Sounds stop and start. Rhythmic sounds may have durations under 1 second, and the hf components of these can have momentary (but repeated) levels as high as the largest bass signals. In this way, music can have low hf power, but high hf energy. Music’s HF dynamics are most apparent in PA and recording monitoring, and on the more lovingly mastered digital recordings. In most vinyl recordings, the hf dynamics have been compressed, to avoid badly designed pickup arms jumping out of the groove. For PA and recording, the ability to capture hf dynamics up to 20kHz has been aided by capacitor microphones (which can have a far smoother, higher-extending, and less compressed response at hf than most dynamic kinds), and on stages, by active mic splitters. These buffer (strengthen) the signals emerging from each microphone, preventing subsequent losses in long cables.

Ultrasonic pleasure While human conscious hearing stops around 20kHz, higher, ultrasonic frequencies in music, up to at least 80kHz, can be perceived by the brain. This much has been long established by listening tests carried out by veteran recording mix-console designer Rupert Neve and producer and monitoring system researcher Philip Newell, amongst others. When frequencies in programme that are above 20kHz are filtered out, sensitive listeners notice a lack of vitality. More recently, it has been demonstrated ‘objectively’, in the sense that specific neural activity and chemical production has been measured [16], that the subliminal perception of the ultrasonic sounds associated with music enhances pleasure.

System limits In conventional digital recordings thus far, being under 18 bits, frequencies above 20kHz have to be wholly absent (or at least heavily filtered) for anti-aliasing purposes. By contrast, moving coil cartridges are well known for retrieving ultrasonic content up to 200kHz or more, off vinyl disc recordings. Some moving coil and capacitor mics are similarly responsive above 20kHz.

Perspective Putting ultrasonic frequencies into perspective, their range is not that remarkable in our hearing’s logarithmic terms, with 200kHz only reaching three and-a-half octaves above the average conscious hearing limit of about 17kHz. 200kHz is also the frequency for BBC Radio 4 in the middle of the longwave band used for public radio broadcasting in Britain and Europe. Unwanted reception is a problem for audio power amplifier design that we will meet in Chapter 3.

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Introduction and fundamentals

References 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Atkinson, John, As we see it – I wish, Stereophile, Feb 1993. Rosenberg, Harvey, The Search for Musical Ecstasy, Book 1 In the home, 1993, Image Marketing Group, USA, ISBN 1-884250-01-7. Duncan, Ben, Black Box column, Investigating a rave, Hi-Fi News, September 1995. Heyser, Richard, A view through different windows, Audio, Feb 1979. Heyser, Richard, Geometry of sound perception, AES preprint 1009, 51st convention, May 1975. Heyser, Richard, Catastrophe theory and its effect on audio, parts 1 to 3, March, April & May 1979. Duncan, Ben, Earlash, Studio Sound, June 1995. Pirsig, Robert.M, Lila – an inquiry into morals, Black Swan/Bantam Press, 1991. ISBN 0-552-99504-5. Black, Richard, A ‘subjectivist’ writes, HFN/RR, Dec 1988. Hawksford, Malcolm, The Essex echo, Hi-Fi News, Aug 1985; Aug & Oct 1986; & Feb 1987. Duncan, Ben, Loudspeaker cable differences, Proc.IOA Vol.17, part 7, 1995. Bohm, David, Wholeness and the Implicate Order, Routledge & Kegan Paul, London, 1980. Duncan, Ben, The spirit of bass, EW+WW, Feb 1993. Colloms, Martin, Basso Profundo – bass perception and low frequency reproduction, Stereophile, Dec 1991. Stuart, J.R, An approach to audio amplifier design, part 1, Wireless World, August 1973. Ohashi, T, E.Nishina, N.Kawai, Y.Fuwamoto & M.Imai, High frequency sound above the audible range affects brain activity and sound perception, 91st AES preprint 3207 and JAES Dec 1991.

Further reading 17 Atkinson, John, and Will Hammond, Music, fractals and listening tests, Stereophile, Nov 1990. 18 Colloms, Martin, Pace, Rhythm and dynamics, Stereophile, Nov 1992. 19 English, Jack, The sonic bridge – understanding audio jargon, Parts 1 & 2, Stereophile, May & June 1993. 20 Harley, Robert, Looking through a glass clearly, Stereophile, March 1992. 21 Holt, J.Gordon, Sounds like ?, Stereophile, July, August & Sept 1993. 22 Schroeder, Manfred.R, Self similarity and fractals in science and art, J.AES, Vol.37, Oct 1989. 23 Zuckerkandl, Sound and Symbol – Music & the External World, Princetown University Press, 1969.

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2 Overview of Global Requirements

2.1 Common formats for power amps Audio power amplifiers are most commonly encountered in one of six formats, which exist to meet real requirements. In order of generally increasing complexity, these are: 1 A monoblock or single channel amplifier. Users are mostly audiophiles who require physical independence as well as implicit electrical isolation (cf.3); or else musicians needing clean, ‘mono’ instrument amplification (Figure 2.1). 2a A stereo or two channel unit. This is the almost universal configuration. In domestic, recording studio ‘nearfield’ and home studio monitoring use, the application is stereo. For professional studios, and for PA, the two channels may be handling different frequency bands, or the same bands for other speakers, but usually it is the same ‘stereo channel’, as amplifiers are normally behind, over or underneath the L, R or centre speaker cabs they are driving. 2b Dual monoblock – as 2 but the two channels are electrically separated and isolated from each other – the intention being so they can handle vastly different signals without risk of mutual interference. However, being in proximity in a single enclosure and possibly employing a common mains cable, together with having unbalanced inputs, inevitably allows some form of crosstalk through voltage-drop superimposition; and magnetic and/or electrostatic coupling and interaction, between wiring. 3 Multi-channel – most often 3,4 or 6 channels. Originally for professional touring use, for compactness, eventually working within the constraints of the 19" wide ‘rack-mount’ casing system, the de-facto amplifier casing standard for pro audio gear worldwide. Three and six channel (Figure 2.2) mono and stereo ‘Tri-amp’ units have been made so the three frequency bands needed to drive many activelyconfigured PA speakers, can come from a single amplifier box. Multichannel power amps are also applicable to home cinema and home or other installed Ambisonic (higher-dimensional) systems.

Overview of global requirements Figure 2.1 The Otis Power Station,a monoblock allpurpose musician’s hi-fi, monitoring, PA and instrument amplifier, half-rack 1u. The soft clip LED indicator is discussed in chapter 3. One unit resides in a Japanese museum of classic Anglo-American rock’n’roll technology. Copyright and design by Jerry Mead, 1988.

Figure 2.2 The Turbosound TMA-23, team designed in 1982, was an early realisation of a high power, rack-mounting stereo tri-amp for quick-connection to the first full-range, touring PA speaker boxes. Note Lemo input connector (centre) and two EP6 speaker outlets, one in use. From the author’s collection.

4 Integrated power-amp + preamp. Not to be confused with monolithic integrated circuits (ICs), this is the familiar, conventional, budget domestic Hi-Fi ‘amp’. The control functions are built in, saving the cost of a separate pre-amplifier in another box. But sensitive circuitry (such as high gain disc and tape inputs) may not sit comfortably alongside the stronger AC magnetic fields commonly radiated by power amplifiers’ transformers and supply and output wiring. Careful design is needed to reap cost savings without ending up with irreducible hum and degraded sound quality. In practice, most integrated amplifiers are built because of a tight budget, and so amplifier performance is traded off in any event. But some high grade examples exist and the trend is increasing at the time of writing. 5 ‘Powered’. The power amp(s) is/are built into the speaker cabinet, to form a ‘Powered’ or ‘Active cabinet’ (Figure 2.3). This approach has been slow to catch on. It has seen some niche use in the past 20 years in smaller installations, and in the home, usually in conjunction with an ‘on-board’ active crossover. Having one or more amplifiers potentially within inches of the loudspeaker parts they are driving has the clear advantage that the losses, errors and weight in speaker cables are brought down towards the minimum. This is most helpful in large systems where speaker Figure 2.3 The rear of a well known active loudspeaker, used both in studios and domestic systems, showing the exterior of the integral amplifier, design by Tim Isaac. Note XLR input and ‘bull bars’, useful to ‘tiewrap’ the cables to. Power amplifiers within active speakers can benefit greatly from the well-defined load. (Courtesy ATC Ltd.)

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2.2 Loudspeakers cables are most often at their longest. Since an amplifier in a speaker cab does not need its own casing, there can be savings in cost, and the total system weight (of amps + speakers) can also be reduced. In the home, the need to live with the conventional amplifier’s bulky metal box is avoided. One downside, at least for touring, is that even if there is an overall weight reduction, the speaker cabinets assume added weight, which may cause flying (hanging) restrictions. There’s the need to runs mains cables as well as signal cables to each speaker cabinet. This is more of a nuisance in large systems. For touring sound, health and safety legislation is also unwelcoming to powered cabs, particularly when flown, on several counts. Also, if flown, maintenance can be onerous and adjustment impossible without remote control. Although beyond the remit of this book, it is worth noting that musician’s ‘combo’ amplifiers are an older, simpler and far more widespread variant of the powered cab.

2.2 Loudspeakers Knowing a little about loudspeakers, the load that is audio amplifiers’ raison d’etre, is a pre-requisite to understanding amplifiers. In the following sections – indeed most of the rest of this chapter those features of loudspeakers that most define or affect the design and specification of power amplifiers are introduced. In subsequent chapters, some aspects of speakers’ behaviour are covered in greater depth.

2.2.1 Loudspeaker drive-unit basics There are six main types of speaker drive-units or drivers used for quality audio reproduction. Another name for a driver is a transducer, a reminder that they transduce electric energy into acoustic energy, via mechanical energy.

Cone drivers The most universal, everyday form of the ‘moving coil’ or ‘electro-dynamic’ type of drive-unit (Figure 2.4 ) has a moving cone, with a neatly wound coil of wire (the ‘voice coil’) attached to its rear. The coil has to be connected to and driven by an amplifier. The coil sits in a powerful magnetic field, and can move back and forth without rubbing against anything. When driven, a signal-varying, counteractive magnetic field is set up, causing the coil and the attached cone to vibrate in sympathy with (as an analogue of) the driving signal. The principles are akin to an electric motor, except that the vibration is linear (‘in and out’), rather than rotational. Moving coil drive-units can be made in many ways. Most have to be mounted in some kind of enclosure before they can be used. Drive unit size (strictly, the piston diameter) broadly defines frequency range. Most cone drivers range from 1" (25mm) up to 24" (0.6m) in diameter, for use at high treble down to low bass. There are at least 15,000 different types of cone materials, textures and weights available. Most are made of paper pulp, but plastics, metals, composite materials, and laminated combinations are also used. Every one sounds different, and measures differently.

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Overview of global requirements Figure 2.4 A moving coil drive-unit, with key parts identified. (Courtesy Funktion One Research) Suspension (‘Surround’) Suspension (‘Spider’) Cone

Gasket Flexible ‘Lead-Outs’

Voice Coil Dust Cap Terminals

Basket Top Plate

Magnet (Ceramic)

Polepiece

There are as many permutations again for the voice coil’s diameter, height and wire gauge; the type of dust cap, magnet, the chassis, and the flexible jointing, called the surround (at the front), and centering device at the rear, the suspension or spider. With all the moving parts, ruggedness and stiffness is pitted against the need for agility, hence levity. This is the main reason why the radiating part is cone shaped. This shape can stiffen the most limp paper against the axial force applied to it by movements of the voice coil.

Compression drivers The second most common type of driver, at least in professional sound, is the compression driver (Figure 2.5). This is simply a specialised form of moving coil driveunit. The depth of the cone is replaced by a much shallower, and usually oppositefacing and dome shaped radiating surface, called the diaphragm. The voice-coil is attached peripherally between the edge of the dome and the suspension. This type is made for some midrange but mainly hf speakers which are horn loaded . All bassbins and most midrange horns employ specially adapted but ordinary-looking cone drivers; these alone can handle the larger excursions required. A compression driver cannot handle more than very small excursions. To avoid large excursions and potential ripping of the diaphragm, a compression driver must never be driven with program having frequencies below its rated range, and not driven without being attached to a suitable horn. Usually, the diaphragm is pressed out of a plastic film, or a phenolic resin-impregnated cloth or other composite, or from very light, but stiff metal, usually aluminium, else titanium or beryllium. Size ranges from about 6" (150mm) for midrange, down to 1" (25mm) for high treble and above.

Soft and hard dome drivers The equal-second most common type of speaker drive-unit is familiar enough: It has an almost hemispherical diaphragm shaped like some compression drivers’ but the dome is forward (like a fried egg) and nearly always working into free air. This is used on its own, instead of a small diameter cone, as a tweeter (hf drive unit). The material can be any of those used in cone or compression drivers.

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2.2 Loudspeakers Figure 2.5 Two types of hf compression driver made by Emilar, widely used in PA systems from the mid 70s.

Magnet Phasing Plug Assembly Back Plate

Voice Coil Diaphragm Assembly

Common voice coil The three types of drive units discussed so far all have similar voice coils. They may range widely in weight, diameter (from 0.75"/19mm up to 6"/150mm) and power handling (commonly from 3 watts to 1000w), but they will all mostly have a DC resistance of five to ten ohms, and a nominal (AC, 400Hz) impedance of 8, 15 or 16 ohms.

The ribbon driver The ribbon speaker is a fourth kind of electro-dynamic drive-unit. Instead of a voice coil attached to the radiating part, the amplifier signal is connected across a length of flat (planar) conductor foil or ‘ribbon’, which is again placed in a magnetic field like a voice coil, but also radiates sound like a cone, diaphragm or dome. Compared to ordinary voice coils, this arrangement can be lighter and certainly presents a much purer (‘resistive’) impedance to the amplifier. The classic ribbon had a very low DC resistance, and was transformer coupled. Modern ribbon speakers have longer strips, amounting to 3 or 5 ohms of near pure resistance, benign to most audio amplifiers connected to it. When ‘built big’ as a panel loudspeaker, a ribbon drive-unit forms a wide-range loudspeaker in its own right, i.e. no cabinet required. There is little breakup in the ribbons’ surface to mar the sonic quality. And, unlike other drive units, absence of a cabinet means the sound source radiates as a dipole, i.e. from both sides. This can be important to the amplifier, in so far as room interaction can change the impedance seen, by reflection. Small ribbon drive-units are used as tweeters. They may be horn loaded to magnify their rather low output. Sonic quality can be very high, although naturally favouring the reproduction of stringed instruments.

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Overview of global requirements The electrostatic source The Electrostatic Loudspeaker (ESL) employs the inverse or dual principle of the electro-dynamic or ‘motor’ types of drive-units that we have just looked at. The movement is provided by electrostatic (electric field) force, rather than magnetic attraction and repulsion. The vibrating part is a thin, critically stretched sheet called the diaphragm. The fixed part, after the capacitor it mimics, is called a plate. Electrostatic drivers are commonly made in the form of panels, like ribbon speakers. A power source (usually from the AC mains) provides the high EHT DC voltage of over 1000 volts, that is needed to polarise the plates. A high signal voltage swing is also required. This, together with isolation from the EHT is attained by interposing a transformer. In practically-sized and costed electrostatic speakers, the transformer and the diaphragm have a surprisingly limited capacity for handling high levels at low frequencies. In primitive designs, overdrive in the bass can cause the diaphragm to short against the opposite plate. In modern ESLs, the diaphragm is insulated. A well know electrostatic employs an aggressive crowbar circuit for protection. If the ESL is subjected to potentially damaging high levels at low enough frequencies, this shorts the speaker’s electrical input, possibly blowing up the amplifier, or at least blowing a fuse or shutting down the music. Under most other conditions, the ESL appears as an almost purely capacitative load, with resistive damping across it.

The piezo driver The two fundamental types of drive unit ‘motor’ looked at so far all date back (in principle) to the early years of this century, or even to the beginnings of the modern harnessing of electricity, two hundred or more years ago. The piezo drive-units’ principle is the dual of the familiar household act of creating large voltages by squeezing crystals. Although piezo-electricity precedes humankind, as it can occur naturally, it has only been widely harnessed in the past 50 or so years, first in crystal mics and pickups, and more recently in fuel-less ‘push button’ gas fire lighting. The dual, or reverse process, that of making a crystal vibrate by applying electricity to it, was first harnessed by Motorola, who have been producing hf drive-units employing this principle since at least 1977. This type of drive unit looks capacitative, rather like an electrostatic, but has a higher DC resistance – so it can draw no long term power. Despite potentially useful high hf performance, since there is still a limited range of piezo drive units, most being fitted to integral, out-dated horn designs, piezo tweeters are not used much in high performance systems, but they are occasionally used in PA systems, and may be found optimally applied in refined custom speaker systems. The Motorola piezo element cannot be ‘burned out’ by too much ‘power’ as it presents a high impedance. But it is rated at about 25v rms, and excess voltage will quickly destroy the crystal. The crystal can even be harmed by room heating. For use with amplifiers having headroom above 25v rms, operating 2 or 3 in series is suggested. This should not degrade damping as it would with a low impedance speaker.

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2.2 Loudspeakers Inductive coupling Eli Boaz at Goodmans, part of the TGI Group (comprising speaker manufacturers Tannoy, Goodmans and Martin Audio in the UK) spearheaded the development of 2-way drive-units where the hf driver is inductively coupled (Inductive Coupling Technology or ICT). It comprises a radiator with a conductive collar (Figure 2.6). Placed within the bass/midrange voice coil, it acts as a single turn transformer, picking up magnetic field most efficiently at hf. This arrangement is limited to use at HF, but there is no need for a crossover, and it is highly rugged – a tweeter that cannot readily burn out. And without having the capacitative loading region of a conventional tweeter (and the load dip of any passive crossover), an ICT driver’s load impedance at HF is benign. Figure 2.6 The exploded hf dome above this Tannoy drive-unit has no ohmic connections, and cannot be burnt out. It employs inductive coupling technology (ICT), the first completely new type of drive-unit to enter mass production for many years. Each new drive-unit type has its loading peculiarities and this adds a new layer of variables to the considerations of amplifier users and designers alike. Courtesy Tannoy Ltd.

2.2.2 Loudspeaker sensitivity vs. efficiency Loudspeaker drive units have to be ‘packaged’ to be usable in the real world. Together, enclosures and drive-units define the efficiency of the resultant loudspeaker. Efficiency (or its derivative, sensitivity) then decides the scale of amplifier power needed. With different high performance loudspeaker types, efficiency varies over an unusually wide range of at least a hundredfold, from about 20% down to 0.2%. Efficiency is not often cited, but can be inferred from the vertical and horizontal polar radiation patterns, the impedance plot, and the sensitivity. Sensitivity is the derivative of efficiency that makers use to specify ‘how much SPL for a given excitation’. In part sensitivity is universally specified because it’s easier to measure. It is given as an SPL with a given input (nearly always 1 watt) at a given distance at close range (1 metre normally). So the spec is the one that reads: ‘Sensitivity 96dB @ 1w @ 1m’. 96dB is a high sensitivity for most domestic speakers but low for professional types. The sensitivity is but a broad measure of efficiency differences, since two factors are missing. One is how the sound energy is spread in space. If it is all focused forwards, sensitivity (dB SPL @ 1w @ 1m) is raised as the sound ‘density’ at the measuring position increases. At low frequencies, rated sensitivity commonly falls as the sound radiation becomes more nearly spherical, while efficiency is unaffected.

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Overview of global requirements Factor two is the impedance. Where mainly resistive, efficiency is about the norm, as computed by integrating the SPL over all the solid angles. But around the (or a) resonant frequency where the impedance changes rapidly from capacitative to inductive, efficiency is high, as little energy is dissipated. With these 4 dimensions of variables (3D space + 1D impedance), converting efficiency into sensitivity figures and vice-versa is not straightforward. But as a rough idea, an 86dB@1w@1m rated domestic speaker is about 0.5% efficient. While with a two sided (planar) speaker, the efficiency might be the same 0.5% but sensitivity would ideally halve towards 80dB.

2.2.3 Loudspeaker enclosure types and efficiencies Horn-loading is by far the most efficient technique. It is between ten and over a hundred times more efficient than any others. ‘Efficiency’ means it gives the most acoustic intensity for a given power input, from the amplifier. Only when a horn (or ‘flare’) is coupled to a transducer with a low output (e.g. a ribbon driver) is the overall efficiency not ‘streets ahead’ of all the other driver+enclosure combinations. The most efficient drivers are the familiar electro-dynamically-driven cone, dome and compression types, particularly those with an optimum balance between the strength of magnetic and electric coupling, the levity of the moving parts and the compliance of the suspension. In the midrange, some ESLs can be as efficient as the cone-driver, both in the context of a refined domestic speaker. The least efficient enclosures are: (i) none (this holds true at low frequencies only), (ii) the sealed box (SB) or ‘infinite baffle’ (IB), and (iii) the transmission line (TL) – used to extend bass response. Of these, the latter two are important, practical forms that have to be lived with. They can in any event be made relatively quite efficient by making the enclosure big. To some extent, Colloms’ law holds here: “Loudness (per watt or volt) is inversely proportional to bandwidth and smoothness”. This is fine until we come to consider the refined horn-speakers which do not attempt 50% efficiency, and where a minimum of three types are needed to cover the audio band. While at least ten times more efficient, there is little or no bandwidth narrowing over ordinary speakers. Compression- and Piezo-drivers are those usually coupled to horns (flares), and may need no other boxing or at least not any specific enclosure, as their rear chamber is usually already sealed. Sound radiation is then mainly defined by the horn, subject to mounting. Ribbon drive units may be also horn mounted; or if ‘Planar’ types, then along with ESLs, they may be simply mounted in a frame that has little effect on the sound radiation which is dipolic, ie. two sided, like a harp’s.

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2.2 Loudspeakers The other two types of drive units – the cone and soft-dome are usually mounted in closed (‘infinite baffle’ or ‘sealed box’) enclosures, or in the case of cone drivers alone, in ported (‘Thiele-small’, ‘vented’ or ‘reflex’) enclosures. Cone drivers are also used ‘coupled to’ horns – either midrange, or bass (‘bins’). In practice, as the rear of the cone’s basket mounting frame is open, and the fragile magnet is also unprotected, cone drivers in bins and horns are always mounted inside the overall enclosure. Horns, transmission lines, sealed and vented boxes, and other loading types may form complete loudspeakers in free permutation. Of these, only combinations of horns or of sealed boxes can cover the full audio frequency and dynamic range within their own family, i.e. without involving each other or the other types.

2.2.4 Loudspeaker configurations: a résumé Few or no single loudspeaker drive units offer overall, high performance audio reproduction. The nearest contender is an ESL panel, which can work as the sole drive-unit for the kinds of music that have no loud, low bass content. But to listen without restriction and risk of damage to every other kind of music, two, three or more drive units must be used to cover low, medium and high frequencies (which from 10Hz to 20kHz span a wavelength range of some 2000 fold !), and over the 120dB+ dynamic range required for high performance sound reproduction.

Matching levels Often, the sensitivities (loudness) of the individual units that are optimum for each frequency band, differ. Commonly, the tweeter is more sensitive than the driver covering bass/mid frequencies. If efficiency is unimportant, the mis-matched sensitivities (which would otherwise cause a uneven, ‘toppy’ frequency response) may be overcome by adding a series ‘padding’ resistor in line with the hf drive unit (tweeter).

Parallel connection Alternatively, in touring PA and wherever else efficiency matters, or wherever high SPL capability is sought, an overall flat response may be attained by using two, parallel connected bass/mid drive-units. Applicability is always subject to coherence in the acoustic result, hence suitable mutual positioning of the paralleled drivers, so they work together. If ‘ordinary’ drivers (i.e. electro-dynamic types), a 15 or 16Ω rating will be likely chosen, so the resultant load is about 8Ω, rather than 4Ω if the paralleled drivers were each the usual 8Ω. Again always subject to coherence in the acoustic result (and thus suitable mutual positioning, generally closer than a quarter of the shortest wavelength), drive units or complete speakers can be paralleled across either a given amplifier; or when this runs out of drive-capability, across amplifiers ad infinitum that are driven with an identical signal, and have either identical, or acoustically justifiable different gains.

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Overview of global requirements Despite the above, the fewer drive-units or speakers reproducing a given programme in a given frequency range, the better the sonic results. As is so often the case in high performance sound reproduction, least is best – if it is usable.

Why crossovers? When two or more drive units are used to cover the audio range, it is usually very important that they only receive programme over their respective, intended frequency ranges. Programme at other frequencies must be omitted, as it will usually degrade a drive-unit’s sonics, by exciting resonances and sub-harmonics. At moderate to high drive levels, physical damage is also likely. The ‘frequency division’ or ‘frequency conscious routing’ that ensures different drivers receive their intended range, and not other frequencies, is the crossover (no relation to crossover distortion). The crossover may also perform phase shifting or signal delay in one or more bands. Principally this is to synchronise (UREI use the phrase ‘time align’) the signals delivered by the drive-units, e.g. because the sound from some has slightly further to travel. A variety of crossover types exist, the more sophisticated designs aiming to have the bands meld neatly in the crossover regions, without boosting or cutting any frequencies.

Crossover point (XOF) The crossover frequency or ‘point’ is chosen by speaker designers on the basis of: (i) power handling. Drops dramatically if XOF too low for the higher driver. Caused by the exponential increase in excursion as LF limits reached. (ii) distortion. For the same reason, rises rapidly for the higher driver below a comfortable XOF. iii) frequency response, both on- and off-axis. If either of these changes abruptly, near the proposed XOF, the XOF had better be moved. But with some driveunit combinations, there is no ideal XOF.

Passive crossovers In the majority of domestic speaker systems, but particularly where low cost or simplicity are paramount, the crossover is passive (unpowered) and operates at ‘high level’, being placed within, and supplied as part of the speaker cabinet (Figure 2.7). Figure 2.7 Passively crossed over cabinet.

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2.2 Loudspeakers In this form, the crossover comprises physically large and heavy, high voltage-rated capacitors, high current-rated inductors (coils), and high power-rated resistors.

Passive low level It is possible (but not very common) to have a passive crossover operating at line level, installed before the signal enters the power amplifier, or otherwise before the power stage (Figure 2.8). The crossover parts can then be smaller and lighter as the voltage and current ratings (that in part determine size) can then be 30 to over 100 times lower. This is a fine arrangement for all-integrated active cabinets. Figure 2.8 Low-level passive crossover.

Otherwise, in ordinary ‘mix and match’ sound systems, one problem arising is that the crossover is physically divorced from the speaker, so careful connection is needed. Another, less daunting, is that any existing, passive high level crossover component values cannot be simply transferred, since they will have been ‘tweaked’ to best suit the vagaries of the drive-units’ impedances.

Active crossovers The active crossover (first suggested by Norman Crowhurst in the 1950s) takes the preceding concept a stage further. Frequency division is accomplished actively. This means using active devices – and a DC power source – to provide filtering in a highly predictable manner. For example, the filters are able to work in an ideal environment, having well defined and resistive loading. This, and the filter function are defined potentially very precisely by active electronics, usually employing high NFB (Figure 2.9). The main disadvantage of active crossover systems is cost, not just of the active crossover, but of the added amplification and cabling. In DIY domestic set-ups there is also the bulk of equipment (if using say three stereo amplifiers, placed centrally; or six monoblock amps and two mono crossovers, half to be placed by each speaker) and their cabling. Such inconvenience is irrelevant in concert sound systems, and even in recording studios. It is also absent in active powered enclosures. Rather the reverse – 1integrated active cabs are ‘Plug in and go’ systems. For everyone else, more care needed when connecting-up an active system, taking care that the drive units are going to be fed their appropriate band. In professional sys-

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Overview of global requirements Figure 2.9 Classic 3 way active crossover.

tems, multipin speaker and line connectors are commonly used so the two or more frequency bands’ connections are always correctly routed. Notable advantages [1] compared to the common, high level passive crossover are: (i) reduced ‘congestion’ and similar intermodulation distortion symptoms as each power amplifier handles only a section of the audio range. (ii) differences in driver sensitivities (considered under ‘matching levels’, above) are ironed out and without compromise, except the requirement for, or use of amplifier headroom, by simply adjusting gain controls. (iii) higher dynamic headroom by diversity, as the programme peaks occurring in the respective out-of-band frequency ranges do not steal any headroom. Also, brief clipping (overdrive) of the bass band (etc) has little effect on clarity when the other bands are not driven into clipping at the same time. (iv) the amplifiers are connected directly to their respective drive-unit(s). There are no reactive components (i.e. the passive crossover) to steal current, but the driveunit’s impedance dips may still demand significant current headroom from the amplifier. (v) the ease and capability of creating highly conjugate, highly matched and closely toleranced crossover functions. Of these, the low-level passive crossover potentially has all the same advantages – except possibly this last item.

Active manifestations Active crossovers commonly take one of three forms. In pro-audio ‘the crossover’ is commonly used packaged in its own box, like other signal processors, as a standalone.

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2.2 Loudspeakers As a power amplifier presents a (literally) well placed opportunity to share a box and a power supply, a few power amp makers offer a pluggable option or ‘octal socket’, where a crossover module or card can be retrofitted. Often, the performance of the card’s filtering (e.g. the frequency and slope rate) may be decided upon by the user. This is often a low-budget option limited to installation work, but while it can save money, it need not be shoddy. If carried out with as much care as any crossover is due, turning an amplifier into a ‘speaker-driving filter’ has the advantage of minimising superfluous hardware and signal path complexity. Otherwise, the active crossover along with the power amplifiers, is placed in the loudspeaker enclosure. This creates an ‘active enclosure’, ‘active speaker’ or ‘Triamp cab’ (if 3 way; else ‘Bi-amped-’, ‘Quad-amped-cab’, etc) that has the advantage of hiding the complexity, and offers a fait accompli, but takes away the flexibility that touring PA users often need. Active systems are widely used in pro-audio, for installed and touring PA systems, and studio control room monitors. In nearly all cases, stand alone active crossovers, discrete power amplifiers, and individual enclosures are brought together. In highend hi-fi, discrete active systems are comparatively rare, except in DIY circles. Active speakers are becoming somewhat more common, at least in the UK and Europe (see Appendix 2).

Bi-wiring Bi-wiring is a ‘part-way house’ to having a low level active (or passive) crossover. A separate speaker connecting wire is provided for each drive unit, or for each frequency band (Figure 2.10). This lessens interaction and intermodulation that is otherwise caused by communal speaker cabling. We will meet this later, under the banner ‘good noding’. Figure 2.10 Bi-wiring improves sonic quality by avoiding superimposition voltage drops over the greater length of the output-stage to speaker connection. Otherwise LF signal currents upset the HF’s driver signal’s purity - and even vice-versa.

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Overview of global requirements Other networks Whether the crossover is passive and high level; or passive and low level; or active, other components may be associated with each drive unit: i) DC protection capacitors are connected in series with drive units to block steady current flow should a DC voltage appear across the box’s, the speaker’s or the driver’s input terminals. They are not required for the hf (and mf) drivers with passive crossovers, where the high and ‘bandpass’ filters already include the required series capacitor – as part of the crossover. In some designs, more complex, active crowbar circuitry is used, in order to obviate the need for a series capacitor. ii) Zobel and other ‘conjugate matching’ networks. Comprising networks of capacitors and resistors, and less often inductors, these act to smooth out the impedance variations of the drive units, either singly or altogether, and as seen by the preceding crossover and also amplifier. iii) Music overdrive protection – in some designs a lightbulb, usually a rugged 12v type, is connected in series with hf drive units which in practice require protection most of all. At worst, the lightbulb will be quicker, easier and cheaper to change than the hf driver or diaphragm. The effect the lightbulb has on sonic quality can be small and sonically benign if the lamp is not visibly glowing during normal loud passages. ‘Auto resetting’ ‘thermal trip devices’, alias Ptc (Positive Temperature Coefficient) thermistors are also used. These are usually in the form of a cement coated disc. At room temperature, they exhibit a low resistance. When eventually tripped by excess current, the hot resistance rapidly increases to about 100 fold, and the protected driver’s power dissipation drops 10,000 fold or pro-rata. The effects on sonic quality of series ptc thermistors are as yet questionable.

Other protection Loudspeakers have also been protected by add-on boxes, containing historic powerreading circuitry for example, which crudely opens a relay in line with the speaker or line level signal if the drive-unit is seen heading towards a burnout. The use of fuses within power amplifiers for speaker drive-unit protection is considered in chapter 5.

2.3 The interrelation of components 2.3.1 What loudspeakers look like to the amplifier There is a tacit presumption that most amplifiers can comfortably drive any ‘reasonable’ loudspeaker. Beyond whatever is ‘reasonable’, discomfort may occur - to all parties. To the power amplifier which is nothing but a loudspeaker driver, the most salient information about any loudspeaker it is expected to drive, and the stress that may engender, is that loudspeaker’s impedance.

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2.3 The interrelation of components Impedances A speaker’s nominal impedance is commonly (and over-simplistically) described by a single round figure, usually 15, 8 or 4 ohms for the majority of moving-coil drive units. With ribbon drive units, or whenever several drive units are paralleled to increase handling or coverage, lower impedances of 3, 2 or 1 ohms or even less are the norm. With electrostatic and piezo (hf) drive-unit types, the load impedance can be higher, but are also more or predominantly capacitative (like a capacitor) across the audio range. This can be far more taxing to the amplifier.

Low vs. high impedances At this juncture it is helpful for those unfamiliar with electronics jargon to grasp a counter-intuitive fact: that the lower impedance, the heavier the (current) loading on the amplifier. To remember this and that 4 ohms is harder to drive than 16 ohms, think of hill slopes: a 1-in-4 hill is far harder to drive or climb up, than a 1-in-16, ie. an impedance in ohms is the reciprocal of the relative loading. Remember also: A low impedance demands more current, and less signal voltage is needed for a given current. A high impedance requires more signal voltage, to be driven with a given current. Figure 2.11 The impedance of a 15” drive unit mounted on a nominal baffle. In some cabinet designs, there could be two or more resonant peaks. Note the labelling of the resistive, capacitative and inductive impedance zones.

>< Resonance

Inductive

Capacitative

Inductive

Resistive

Variation vs. frequency Loudspeakers’ impedances nearly always vary over the frequency range of use. Figure 2.11 shows how a nominal 8 ohm, 15" bass drive-unit typically varies from

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Overview of global requirements 5.5 ohms at 450Hz, peaking up to about 40 ohms or so, at the mechanical resonant frequency, which typically lies between 20 to 120 Hz for a bass driver. Here it is 31Hz. Together, the drive-unit and the speaker enclosure largely determine this. Impedance also rises to a maximum at (and beyond) the highest usable frequency. At and about the resonant frequency, the impedance variation at the loudspeaker’s terminals is due to the reflection of mechanical energy storage, and damping, back to the electrical domain. Figure 2.12 shows that the loading is capacitative on the right side of the resonant peak, where impedance is falling with increasing frequency, while the impedance that slopes upwards with increasing frequency, on the left side of the resonant peak, is inductive. Figure 2.12 shows this in another domain. When the phase (lower graph, left scale) is positive, the impedance is capacitative; when negative, it is inductive. When towards the centre, it is resistive. Dead centre is pure resistance. Figure 2.12 The impedance of the previous figure (upper graph), shown alongside the phase map (lower graph), clearly shows the relationship between pure resistance, and inductive and capacitative phase - at least in terms of voltage. In some instances, a plot of current-phase might be more appropriate.

Resonance IMPEDANCE Inductive

Capacitative

Inductive

Resistive

Inductive

PHASE Inductive - - - resistive - - -

Capacitative

The resonant frequency area(s) of any bass speaker system are commonly highly stressful to amplifiers, and with many BJT amplifiers, when contact is prolonged by a low-enough frequency and perhaps an insistently-enough pounded note, it has frequently been fatal. Other amplifiers have been known to simply burst into uncontrollable oscillation.

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2.3 The interrelation of components At or above the highest usable frequency, the impedance rise is again inductive. It represents the effects of the voice coil inductance. Eddy currents, as such, or manifest as skin effect and proximity effect, may also contribute to the inductivity. Inductive effects are the cause of back EMFs (‘kick back’ voltages) that unless damped can upset sound quality, and can even destroy an unsound amplifier design.

Passive crossover effects Most high performance speakers for domestic and small studio use contain passive (unpowered) crossovers. Such enclosures are driven from a single amplifier. Passive crossover networks are ‘in line with’ the drive units’ impedances. The combination is complex; it may increase or decrease peak current demand, hence loading. As an example of the latter, extra parts may be added to create a conjugate crossover, which makes the overall loading look resistive, but also absorbs power.

Static vs. transient Conventionally, impedance values are taken after applying a steady and repetitive test signal (e.g. continuous sine wave) and allowing a few moments for the recovered signal amplitude that represents impedance, to settle. In the short term, impedance can be considerably lower. At worst [2,3] it is possible for an ordinary dynamic (moving coil) type of loudspeaker to demand current as is it had 1/6th of its nominal impedance. In other words, an 8 ohm speaker can sometimes look like 1.4 ohms. This will not happen all the time or even very often, nor for very long at a time – but for high quality sound reproduction, and not forgetting that music involves repetition, the possibility must be allowed for.

Acoustic contribution The impedance (load) characteristics of drive units can be affected by cabinet air leaks, also by reflections in the room, hence the positioning of the enclosure. Horn loaded drive-units are usually the most sensitive to this. The upshot is that most loudspeaker loads are a wide variable, not just between different models and types, but depending on programme dynamics and excitation frequencies.

2.3.2 What speakers are looking for The fact that most loudspeakers do not employ conjugate impedance compensation, and so they have impedance curves that vary ‘all over’ with frequency, means that for high performance, the amplifier kind the speaker needs to see is a ‘voltage source’.

Why voltage? The signal voltage must be almost unaffected (ideally far below say 1% change) whether the speaker is connected or not; and likewise, regardless of whether it’s drawing 50 milliamps or 50 amperes. That means a ‘stiff power source’, alias a low impedance or ‘high-current-capable’ source.

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Overview of global requirements If the source impedance isn’t low (enough), then as the speaker’s impedance varies with frequency, the change will be superimposed on its own frequency response as a tonal aberration [4, 5]. A source impedance that is almost as high as speakers’ own minimum impedances is a major failing with power amplifiers having low or nil global negative feedback, and the outcome is a tonal anomaly as gross as 5dB [4]. This may not be all bad, but it will certainly be arbitrary.

Energy control In part, a voltage source is required to drive speakers, because loudspeakers store, as well as convert, energy. The fundamental resonance is the place (in the frequency domain) where this is most true. Some of the stored energy ‘kicks back’, and needs to be quickly damped (dissipated) to avoid transient distortion or ‘smearing’. The same reactive effects may also demand surprisingly high peak currents from the amplifier at other times, when driven by music signals. In both cases, the answer is a high current sourcing and sinking capability. Both of these features are implied but neither are guaranteed by a low source impedance. The overall requirement is a ‘current-capable-enough voltage-source’. So far, most power amplifiers throughout history have aimed to be this, but some have come closer than others.

Damping factor? The majority of high performance amplifiers are solid state and employ global (overall) negative feedback, not least for the unit-to-unit consistency it offers over the wild (eg. +/–50%) tolerances of semiconductor parts. One effect of high global NFB (in conventional topologies) is to make the output source impedance (Zo) very low, potentially 100 times lower than the speaker impedance at the amplifier’s output terminals. For example, if the amplifier’s output impedance is 40 milliohms, then the nominal damping factor with an 8 ohm speaker will be 200, ie. 40 milliohms (0.04) is 1/200th of 8Ω. This ‘damping factor’ is essential for accurate control of most speakers. Yet describing an amplifier’s ability to damp a loudspeaker with a single number (called ‘damping factor’) is doubtful [6]. This is true even in active systems where there is no passive crossover with their own energy storage effects, complicating especially dynamic behaviour. Figure 2.13 again takes a sine-swept impedance of an 8 ohm, 15" driver in a nominal box to show how ‘static’ speaker damping varies. Impedance is 70 ohms at resonance but 5.6 ohms at 450Hz. Now, at the bottom, is plotted the output impedance of a power amplifier which has high negative feedback, and thus the source impedance looking up (or into) it is very low (6 milliohms at 100Hz), though increasing monotonically above 1kHz. The traditional, simplistic ‘damping factor’ takes this ideal impedance at a nominal point (say 100Hz), then describes attenuation against an 8 ohm resistor. This gives a damping factor of about 3 orders, ie. 1000, but up to 10,000 at 30Hz. Now look at the middle curve: This is what the amplifier’s damping

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2.3 The interrelation of components Figure 2.13 Views of the damping surface in 2D. Lower plot shows the very low steady-state output source impedance of a typical transistor amplifier with high NFB. The middle plot shows how this degrades after passing down a few metres of reasonably rated cable, and a series capacitor (which might be the simplest crossover, or for fault protection). The upper plot repeats the impedance vs. frequency behaviour of the 15” bass driver. The effective damping factor is the smaller and highly variable difference between the upper and middle plots, not the difference between the highest impedance on the upper plot and the lowest on the lower plot, used by amplifier makers! A View of the Damping Surface What the 15" Driver + Zobel look like at the cabinet terminals

What the Amplifier looks like at the Speaker’s End (inc. Cable & DCP Cap)

Skin Effects

Series Capacitor Cable DC res

NFB Reduction

What the Amplifier looks like – at its output terminals

ability is degraded to, after is has traversed a given speaker cable and passed through an ideal 10,000µF series capacitor, as commonly fitted in many professional cabinets for belt’n’braces DC fault protection. The rise at 1kHz is due to cable resistance, while cable inductance and the series capacitance cause the high and low-end rises respectively above 100 milliohms. We can easily read off static damping against frequency: At 30Hz, it’s about x100. At mid frequencies, about x50, and again, about 100 at 10kHz. However, instantaneous ‘dynamic’ impedance may dip four times lower, while the DC resistance portion of the speaker impedance increases after hard drive, recovering over tens to thousands of milliseconds, depending on whether the drive-unit is a tweeter or a 24" shaker. Even with high NFB, an amplifier’s output impedance will be higher with fewer output transistors, less global feedback, junction heating (if the transistors doing the muscle work are MOS-FETs) and more resistive or inductive (longer/thinner) cabling. Reducing the series DC protection capacitor value so it becomes a passive crossover filter will considerably increase source impedance – even in the pass-band. The ESR (losses) of any series capacitors and inductors will also increase source imped-

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Overview of global requirements ance, with small, but complex, nested variations with drive, temperature, use patterns and aging. The outcome is that the three curves – and the difference between the upper two that is the map of damping factor – writhe unpredictably. Full reality is still more complex, as all loudspeakers comprise a number of complex energy storage/release/exchange sections, some interacting with the room space, and each with the others. The conclusion is that damping factor has more dimensions than one number can convey.

Design interaction While high performance loudspeakers are being designed and optimised, and certainly before they are finalised for production, in-depth listening is a pre-requisite. This means that amplifiers are required to drive them while they are being tested and optimised in the design process. Many drive-unit and speaker manufacturers are limited (or limit themselves) to using just one amplifier make to test their designs and production. The situation is rarely publicised. It follows that many loudspeakers are inevitably looking for that one kind of amplifier that was used when they were ‘voiced’ and ‘tweaked’ by their designer(s). With no less potential for habitual patterns, listeners are looking for the amp that interacts with a loudspeaker in a particular way. The combined behaviour or ‘chemistry’ is complex, and can be unpredictable and frustratingly unrelated to the type or class of amplifier. Here is just one reason why the ideal high performance power amplifier/speaker combination can be determined only by trying them together, and why quite disparate amplifier designs and topologies may shine equally through a particular speaker. Knowledgable ‘high-end’ loudspeaker designers and manufacturing companies employ a well chosen group of different power amplifiers. One or two will be the best sounding ‘references’; some will be widely-used models, and not particularly good performers; others will be niche models, discovered accidentally over the years, that expose loudspeaker problems.

2.3.3 What passive crossovers look like to amplifiers For conventional full range loudspeakers with passive crossovers, the crossover components stand between the amplifier and the speaker. Unless the speaker’s drive unit(s) is/are blown, or have been disconnected or removed, then the crossover won’t usually be ‘seen electrically’ (by the amplifier) on its own. Figure 2.14 shows the impedance that would be seen by the power amplifier if the loudspeakers were 8 ohms resistors, and how it drops from about 10.5 ohms to just over 5 ohms, at the crossover point. As both drivers are being driven at this point, it’s what you might expect. Figure 2.15 shows how much the picture changes when the resistors are replaced by real speakers: now there are two dips. In the crossover’s pass-band where there should be no (attenuative) action on the signal voltage at all, the crossover should be ‘transparent’. In practice, one pass

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2.3 The interrelation of components Figure 2.14 If speakers were simply resistors, the load on the amplifier might appear as here, with the apparent 11 ohm load simply halving to 5.5 ohms at the one point where the two drivers are both drawing substantial current, the crossover point. Note that the crossover dip may as well be a resonance, and as such adds to the amplifier’s load stress.

Impedance of 2 Way Passive Crossover Loading = all 8 ohm resistive

XOF

Figure 2.15

XOF

LF rescue

HF dip

Here, the resistors are replaced by drive units having the impedance characteristics shown in the lower graph. The upper graph shows how the impedance seen by the amplifier has changed - notably two dips where there was one.

Impedance seen through a 2 Way Passive Crossover

Bass

Mid/High

Bass

Individual Drive Unit Impedances

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Overview of global requirements band is another driver’s stop-band. Even a steady state test signal will experience the added reactive loading at most frequencies. In turn, the crossover is liable to add to the peak current demanded by the drive-units. With music signals, a passive crossover stores energy and can ‘kick back’ like a speaker, potentially adding to the speaker’s demands. Overall, for the amplifier’s own good, passive crossovers benefit no less than the drive unit, from being coupled to an amplifier with very low source impedance, with ample current sourcing and sinking capability.

2.4 Behaviour of power amps as voltage sources In the preceding section ‘Damping factor?’, it was mentioned that most high performance audio amplifiers employ some global (overall) negative feedback and that this confers a very low output source impedance, usually well below 100mΩ (9kHz. Conducted onto the AC mains, over 0 Hz (DC) to 2kHz; then 150kHz to 30MHz.

Testing for Immunity to: (50082) Ambient, RF fields, over 27MHz to 500MHz, at 3v/m, unmodulated. * Ambient power frequency magnetic fields, 50Hz, 1 or 3A/m. Electrostatic discharge, 4kV contact to case; 8kV air. Fast transients up the in/out ports, common-mode, up to 500v, at 5kHz. RF up the in/out ports, 80% modulated @ 1kHz, 0.15 to 80MHz at 3v rms, Zs = 150Ω. Mains voltage fluctuation, 30% for 10ms; 50% for 100ms. Mains power interruptions, for 5 secs. Mains voltage sag or surge, +/–10%. Mains borne transients, 2kV common mode, and 1kV differential mode. Fast transients up the mains ports, common-mode, up to 1kV, at 5kHz. Mains, RF, 80% amplitude modulated @ 1kHz, 0.15 to 80MHz at 3v rms, Zs = 150Ω. * This test is valid even though it is not part of the generic standard.

At the time of writing, EMC and also LVD (Safety) testing are in a state of flux. Up to date information should always be sought from, or verified by, experienced EMC and safety consultants, or test houses.

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Real world testing – rationale and procedures

References 1 2 3 4 5 6 7 9

Atkinson, John, The puzzle of perception, Stereophile, Feb 1992. Atkinson, John, and Will Hammond, Music, fractals and listening tests, Stereophile, Nov 1990. Colloms, Martin, Some observations on the results of objective and subjective technical reviewing practice in high fidelity, Proc.IOA, Vol.13, part 7, 1991. Phillips, Wes, Testing, testing, one-two, three, Stereophile, Aug 1994. Atkinson, John, The blind, the double blind and the not-so-blind, Stereophile, Aug 1994. Duncan, Ben, EMC: Testing times?, Studio Sound, Dec 1995. Duncan, Ben, A quick crowbar, HFN/RR, June 1982. Marshman, Chris, The Guide to the EMC directive 89/336/EEC, EPA Press (UK), IEEE press (USA), 1992.

Further reading 10 Colloms, Martin, The sound of amplifiers, TAA, 3/1985, reprinted from HFN/ RR, May 1985. 11 Duncan, Ben, Realistic amplifier testing: Procedures for professional touring music PA, proc. IOA, Vol.16, 1994. 12 Harley, Robert, The listener’s manifesto, Stereophile, Jan 1992. 13 Harley, Robert, Deeper meanings, Stereophile, July 1990. 14 Heyser, Richard.C, Hearing Vs. measurement, Audio (USA), Mar 1978.

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9 Choice, application installation and set-up ‘On the big screen they showed us the sun, But not as bright in life as the real one...’ Lyrics by Bernie Taupin.

9.1 Manufactured goods, a résumé “A manufacturer not only has to design a worthy product; they have to be able to make it consistently and reliably. Each is important as the other, and both are important to users.” John Atkinson [1] High performance sound systems can cost the individual or company alike more than their home, studio or warehouse. Now more than ever before, intelligent purchasers in Western countries are wary of purchasing pretentiously advertised, over-packaged, gleaming goods that will consume the disposable income or equipment budget for some months or years while having a high likelihood of failing to perform, or becoming boring, or ruinously expensive to maintain after a while. The ‘secret’ is not just that most high performance audio equipment is built to last a lifetime or more, but that it has a value quite unlike that of other material objects, as a tool that establishes a link or tunnel with higher realms. After you have established the quality infrastructure, the idea is to spend the money on exploring what (musically) can come down that tunnel. Similarly upfront hi-tech tools giving access to higher consciousness include immersion chambers, spaceflight, and the gear of extreme physical sport. That many manufactured items have a short life, or are unsuited to the purpose which their marketing image pretends they meet, is not a new problem. We can reasonably presume that Roman citizens complained about it two millennia ago. Certainly, over a century ago, one of Britain’s arch-mentors of quality, wrote: “There is hardly anything in the world that some man can’t make just a little worse and sell just a little cheaper, and the people who buy on price alone are this man’s lawful prey.” John Ruskin, 1819-1900

Choice, application installation and set-up Fortunately, the dedicated users and the more intelligent reviewers of audio power amplifiers have become good at assessing quality, in all areas. So the frequent comment by those who are astute about quality in everyday life, “They don’t make stuff like this any longer” when surveying surviving 40 year old artifacts against many modern goods, may be untrue when it comes to power amps. The reverse may actually be true. The development of domestic high performance audio has undoubtedly benefited from the continuing, public scrutiny of product quality by experienced reviewers, in magazines. One of the most important discoveries of the second part of the 20th century is that while quality is hard to define, humans have intrinsic quality sensors that precede intellect. “Reality ... is the moment of vision before the intellectualisation takes place. There is no other reality.” And “..what is good... and what is not good, need we ask anyone to tell us these things ?” Robert Pirsig, in Zen and the Art of Motorcycle Maintenance [2] Listening to equipment is an example of this skill being honed. But in any manufactured item, quality is more than this, and the skill of ‘picking a good one’ and ‘by instinct’, is there for the using.

9.1.1 Choosing the right power amp, domestic Many domestic power amplifiers are of the ‘integrated’ (pre+power) type, but a minority of these may be described as high performance. A ‘separate’, ie. a discrete power amp, as used by dedicated listeners and professionals, reduces the number of opposing performance and feature compromises, and increases the scope for future changes.

How much power ? Be aware that even if the cited watts are ‘honest’, the relation between wattage figures and loudness is counter-intuitive. Most notably, it is under-powered amplifiers that are more likely to burn-out HF drive units. Specifying the ‘right wattage’, a power rating that will satisfy the listener and also suit the speakers without risking ‘blowing them up’, is complicated by the uncertainty that ‘apples are reallybeing compared with apples’.

Programme ratings For example, most low-budget hi-fi speakers are already rated (in watts) for ‘programme’ (music), so the power amplifier’s, average watts per channel rating shouldn’t range much above this. A power capability up to twice the speaker’s ratings is likely to be harmless if programme material always has reasonably wide dynamics and no sustained high levels above 5kHz, i.e. avoid playing highly compressed sounds, and raw percussion and synths, at high levels. Then a 200 to 300w/ch amplifier would be the most for speakers rated at 100 watts to 150 watts programme.

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9.1 Manufactured goods With more honestly rated, possibly more expensive loudspeakers, the rating given may be a continuous type, based on the AES/EIA test. If so, programme ratings will be x3 to x10 greater. So a 60w AES rated enclosure will handle programme up to from 180 to 600 watts, provided it is not compressed, and that the amplifier is not driven into clip other than occasionally and momentarily.

Power rating truth The amplifier wattage needed for a given loudness depends a little on how near you are to the speakers, and a lot more (often) on how sensitive the speakers are. The loudness difference between the least and most sensitive Hi-Fi Speakers is up to at least 8 fold for a given drive level. This represents a 1000 fold difference in power conversion efficiency, alias a 30dB difference in SPL. Just changing to a 3dB more sensitive speaker (e.g. from 90dBSPL @1w @1m to 93dBSPL @1w @1m) is equal to doubling the amplifier power rating.

Scaling Increased watts give only inverse or anti-logarithmic (read: slack) increases in sound level, yet cost linear money. A 500 watt amp and speaker will both cost getting on for ten times more than 50 watts. But a 10 times more efficient speaker need not cost anything like 10 times more.

Power satisfaction The easiest and most certain way to know what power rating is required is to forget about precise power ratings and listen with the prospective ‘bride’ speakers that the amplifiers under selection may be mated with. The maximum useful loudness can be quite different to expectations, and may depend on the music being played, according to whether the amplifier has output protection, and whether this is being triggered by the speaker’s dynamic load. Active systems (bi or tri-amped) are well known for producing clean sound levels higher than the simple aggregation of the four or six (or more) amplifiers channels’ power ratings would suggest.

High level defects Another cause of inadequate or unsatisfactory loudness (e.g. where 100w of power amplifier Y sounds quieter than 30w of amplifier Z) occurs when the sonic quality degrades half a dB or more below clipping. A well-designed power amplifier using NFB maintains its sonic character up to just below clip, say within 0.2dB. But above clip, the distortion sets in extremely abruptly and if prolonged, soon reaches unbearably high levels. Some listeners prefer to accept ‘gracefully decaying’ sonic performance in the 10 to 3dB leading up to overload. Called ‘soft clip’, this is second best to adequate headroom. Yet even the high-end industry and the informed public have a historic and continuing avoidance of high headroom amplification, in part through fear of the high power numbers (One Thousand watts and above) that adequate

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Choice, application installation and set-up headroom confers, and partly through a belief that such high powers (which actually have only up to an order higher voltage and current swing than conventional designs) must necessarily degrade sonic quality.

Blind specification If buying without listening (or being able to listen) to how loud the amp will go with the speakers it will be partnered with, you will be reliant on a power figure. However notional this is, at least check what impedance a prospective amplifier’s power is developed into. The issue is not that higher powers are always developed into lower impedances than your speaker has, but that the advertised or brochure’s ‘headline’ power rating, is the one that is developed into the nominal impedance of your speakers. In other words, a specified ‘100 watts’ won’t be available into your 16 ohm speakers if the 100 watts is only developed into 2Ω. Only about 13 watts will be available. Misreading − or not reading − the maker’s (or a good technical reviewers’) specification, then not listening, could leave you paying dearly for hypothetical power and headroom.

Quality Somewhere along the line, decor and aesthetics will ultimately matter to someone, and likely more so with physically bigger units, particularly if it (or they) will have to be positioned dominantly as a ‘room sculpture’. Nothing short of a physical inspection and a relaxed period of hands-on use can allow quality to be perceived in an apposite perspective − and all the more so for astute and skilled purchasers. Reliability must somehow be gauged. Parts inside most power amplifiers will be subjected to more stress than any other item of Hi-Fi equipment. Most domestic power amplifiers are still heavy, so the cost in carriage can be substantial if they have to be sent back for repair. But then an amplifier in a sturdy case will more likely be built electrically sturdily, so the likelihood of the need is less!

Rationalised controls Domestic power amplifiers are usually even more bereft of unnecessary or tangential features than utilitarian pro-PA amplifiers, features that regularly appear on massmarket, Mid-Fi domestic and low-budget DJ-grade power amplifiers, most notably gain controls, ‘swing needle’ (or LED) meters indicating ‘power’ output , and any other kind of superfluous LED indication. The money saved can potentially be used to increase real quality. However this should not be taken too far.

Recommended features A clip indicator LED is invaluable (with conventional solid-state amplifiers having high global NFB) for knowing for certain when you are exceeding limits that may quickly burn out part(s) of your speaker system. Even if you have enough headroom so you never see it light in the loudest passages, how could you be sure without it? The clipping of amplifiers employing valves and/or low NFB is less abrupt, far softer and far less likely to damage drive-units.

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9.1 Manufactured goods A ‘we have a problem’ ‘Error’ or ‘Protect’ LED is most useful in the eventually probable event that something is (or goes) wrong, at least to save you wasting time and energy trying to work out why there is no sound. A second set of parallel output terminals or sockets, for bi-wiring. Headphone sockets are rarely seen on any modern power amplifier, excepting integrated types. For most users, they are superfluous. Fortunately, if required, usually for when you want to use medium-to-high impedance (rated at 100 ohms or greater) headphones, it is straightforward to add (or have added) a headphone socket. Switching of speakers is best avoided. An advantage of using the professional Speakon speaker connectors in that speakers can be disabled (and the plug contacts cleaned) without removal, by simply twisting. Low impedance (50 ohms or lower) headphones should never be plugged into an amplifier capable of more than a a few tens of watts, as both the headphones and the wearer’s ears stand a chance of being traumatised if anything like full level is passed more than momentarily.

9.1.2 Choosing the right power amp, for pro users In touring PA, reliability, low weight, a certain fashionable ‘u’ size for a certain power, and ability to ‘kick ass’, have often taken precedence over general sonic quality in any selection listing. In recording studios, there is more chance of the sonic results being the priority, yet the amp may be ‘chosen’ by playing safe and buying on a name or untested recommendation alone; or the monitor speakers’ maker or installer may specify what’s to be used. The latter may be the best route, but the suitability of such a specification should be demonstrable by listening and comparison.

Power The average power rating is commonly selected to be higher than the AES/EIA or ‘rms’ or ‘average continuous’ power ratings cited by most professional drive-unit makers. Provided clipping does not occur, or only happens occasionally and very briefly, both the speaker and in practice, the amplifier power ratings may safely be three to ten times this. For the speaker, this rating over the continuous is well known as ‘Programme Power’. This procedure naturally allows a given amount of hardware to be used to its fullest. This is important not just for operating costs, but also for sonic quality, since it allows the number of sound sources to be minimised. Unless there is severe compression (e.g. as might be caused by over-processing, or by clipping back at the mixer), the amplifier’s unclipped maximum power delivery will be largely in reserve.

Bridging With a cursory inspection, bridging sounds like a thoroughly good thing. It is after all balanced drive. But it’s commonly overlooked that bridging halves any given output damping, doubles the amount of circuitry, internal connectors and solder

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Choice, application installation and set-up joints in the signal path, and perhaps most important, almost wholly cancels all even-order harmonics created in the amplifier. As no amplifier has solely even harmonics, odd harmonics are left, and most are highly unpleasant and fatiguing [3].

Bridge frustrations Usually, rated power is barely doubled. Amplifiers capable of about 1kW into 8 ohms when bridged are relatively commonplace and ‘old hat’. Bridging makes a nonsense of precious power-to-size ratios: 2 channels of 1kW into 4 ohms become 1 channel of 1.2kW into 8 ohms, without any halving in size to show for it ! In its defence, bridging is fine if it does the job, but the potential cost in size, weight and sonics must be evaluated carefully. On the other hand, bridging ideally reduces supply rail noise intrusion, and can be used to build more reliable amplifiers, where one half of the bridge continues with only a modest volume reduction, if the other side is sick or being repaired. Overall, bridging two amplifier channels remains a mixture of good and bad in engineering and sonics, a frustration to the designer and the user alike.

High power Bridged or not, amplifiers that produce above 1kW/ch into 8 ohms/ch or above 2kW/bridged are increasingly available. In PA, appropriate use of these multi-kW per-channel amplifiers with the new class of drivers and suitable enclosures will reduce the number of sources, help simplify acoustic calculations and the related predictive accuracy, improve sonic quality, reduce rigging time, reduce costs and potentially increases system reliability, as there is simply less kit to go wrong.

Sonic chemistry Some amplifiers sound good whatever they’re used with. With others, reliability and sonics may be considerably influenced by the boxes they are driving. An amplifier that sounds poor or shuts-down driving one type of cabinet, may shine with a different kind − even one working over the same frequency band. Amplifiers that measure badly can sound good to some users, but inaccurate to others. Pleasant amplifier distortion can mask unpleasant distortion elsewhere in the system, but if the whole system omits unpleasant distortion, the masking is unmasked. This means if you choose a given amplifier on the reasonable basis that it sounds best in your PA or studio, you may find that a completely different amplifier is preferred when or should you change the mixer or other major components.

Listening test caveats How well a given amplifier sounds on the day you check it out may depend on whether it was warmed up, the quality of the mains supply, and the general health of the system. For example, if unbeknown to you, one channel of the mixer source happened to be marginally stable and putting RF at 1MHz onto the signal, then Amp

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9.1 Manufactured goods ‘B’ with better RF filtering might win over Amp ‘A’, which has lesser filtering, and would otherwise sound better.

Reliability tips While reliability can be estimated by seeking out the experiences of other owners, inspection of the construction, and asking the maker specific questions, there is no way to be absolutely certain. Production of any previously reliable amplifier model can prove unreliable in use at any point in the future if key factory staff are lost, or change, or if the design is ‘re-engineered’ by accountants. And even a well-engineered, generally rock-solid design can fail if a particular user and their equipment accidentally uncovers out some weak spot. Equally some users are happy with, and get reliable use from, designs that can be readily blown up when bench tested. An amplifier that blows up when driven continuously into a 2Ω load is a non-problem if the amplifier is only driven with music and driven into 8Ω speakers that do not exhibit significant impedance dipping.

Transistor reliability With good protection, and otherwise solid design, it is the junction temperature and temperature cycling ‘world line’ of the power transistors, that have the foremost effect on audio power amplifiers’ reliability. Like aeroplanes, power devices have a finite number of cycles before metal fatigues, while every 10°c rise in temperature approximately halves a semiconductor’s lifespan, through chemical diffusion. Considering amplifier cooling, the large (and especially full depth) cooling tunnels seen in many high power amps are less potent than they seem. Frequently, when working hard, the temperature of the hottest device at the outlet end of the tunnel, can be as much as 20°C hotter than the one by the air inlet (Figure 9.1, overleaf). And this is just the surface temperature. Internal, junction temperature differences can be greater, especially if the transistor fixings differ in tightness. Such temperature differentials confuse power sharing and will lead to unreliability as the hotter devices blow prematurely. An amplifier suffering significant temperature differentials between the output devices (whether this is caused by careless cooling design; or by poor mounting of devices) should be derated for reliability, to well below the potential power capability. Because the maker can only derate so far while meeting a salable price tag, some trade-off in reliability and sonics is inevitable. A number of modern designs (e.g. specific models by ARX, BGW, C-Audio, Chevin, Crown and MC2 Audio) employ a short but wide lateral heat-exchangers cooled by a plane airfront. With all the power devices exposed equally, they operate at near identical junction temperatures, sharing out the thermal stress equally (Figure 9.2). This approach allows every device to be safely used to its full rating, and fewer devices (and less stringent protection) may be needed as a result, for a given level of

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Choice, application installation and set-up Figure 9.1 The power devices on blown, tunnel heatsinks rarely operate at the same temperature. This degrades reliability and sonic quality and wastes precious silicon resource. © Studio Sound

Figure 9.2 On lateral heatsinks, power device temperatures can be almost perfectly equal, so thermal stress is experienced equally. © Studio Sound

power handling and reliability. Suitably designed short cooling tunnels can also offer isothermal operation. While one cannot really estimate the reliability of an amplifier by counting the number of devices alone, a broadside-cooled array of power transistors is a good point you can tell about an amplifier, just by taking the lid off. Users can contribute greatly to reliability by shielding amplifiers from stressful occurrences, e.g. see section 9.7.

Mains universality In territories where the AC mains differs from that in the maker’s territory, users (or importers) should carefully evaluate and monitor the maker’s success at the conversion. For example, the transformers in USA made equipment are designed for a nominal 60Hz and 110 to 115v AC. In the UK, the supply is not double as is often

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9.1 Manufactured goods presumed, but 240 volts and often up to 256 volts (irrespective of theoretical EU normalization). Also, the frequency is 50Hz. Alone or together, the lower frequency and higher voltage will cause a transformer designed down to the last cent for 60Hz, and down to an inadequate primary voltage, to run hot, buzz loudly, and possibly fail at an early stage. Sonic quality may also be affected by line frequency, at bass frequencies in particular, as power supply energy replenishment is naturally less frequent at 50Hz (50 cycles per second), vs. 60Kz. Users operating internationally should also check that the amplifier’s power supply is suited to use on other mains supply voltages and frequencies, and conservatively rated, not forgetting that mains voltages outside of the UK and Western Europe are not so standardised and/or not nearly so tightly adhered to both in the ‘3rd world’ and in the rural outback of spacious countries like the USA, Canada and Australia.

Listening Relaxed listening is a good start (forget A-B tests unless you have an strong ‘photographic’ short-term memory of music). Make the repertoire extensive; listen over a few days, with lots of different source equipment, different speaker boxes and different people. If you care about checking in on finer differences, make a point of listening in stereo. This applies even for PA, and even if stereo content in concert PA has mostly so far been minimal. If you’re looking for a bass amplifier (say), and one of the shortlisted amps you are trying is obviously not giving the performance you were after, it might be worth briefly trying it in other roles. This is the way great discoveries are made ! Equally, if you experience disappointing results, it’s valuable to contact the dealer or maker. Getting them to supply another sample unit and making doubly sure that you’re using it optimally may make no difference, but equally, the tables have been turned before on such occasions.

Mechanics A mechanical layout that spreads the load can make a big difference to the comfort, safety and longevity of a touring power amplifier. Examples of bad mechanical design include those amplifiers with: (i) Much or most of the weight focused in any one area away from the centre other than towards the front and symmetrically about the centre-line. (ii) More than about 6"/150mm deep without rear rack supports. (iii) Sharp (un-radiused) metal edges, liable to cut or injure installers. Good designs may include such features as: (i) Recessing of vulnerable or critical controls or switches. (ii) A single secure but easily removable cover giving access to all the breakable or wear-prone parts most requiring attention. (iii) All air-spaced PCBs, allowing rapid drying out in the event of flooding.

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Choice, application installation and set-up Individual reality Overall, power amplifiers are like other machinery that humans experience intimately: their ‘world-line’ interaction with your system in your part of the universe is not predictable from any spec sheet. Even with in-depth technical understanding, no amount of technical data and bulleted, internal technical features can tell you anything definite or specific about the unmeasurable qualities. Therefore you may find it useful to do the listening tests before reading any spec sheets and technical papers, or dissecting or abuse-proving the unit under evaluation. If the amplifier sounds bad, you never have to wade that far.

9.2 Howlers Here are some common and ‘it really happened’ defects to look for in manufactured units. Based on hundreds of reviews of both domestic and professional power amplifiers costing from £150 up to £5000, or even more.

Illuminations * * *

‘Clip’, ‘Peak’, ‘Error’ or 0dB bargraph LEDs that warn of overload more than ½dB early or far worse, more than ¼dB late. ‘Clip’, ‘Peak’, ‘Error’ or 0dB bargraph LEDs, where the accuracy of clip indication depends on the speaker loading and mains voltage. Important panel LEDs that are invisible unless viewer’s eyes are almost on-axis.

Noises * * *

Transformers that buzz audibly with normal mains harmonic variations. Overly noisy fans for domestic and some PA applications. Hum at output with no input connection.

Anti-social behaviour * * * * * * * *

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High inrush current at switch-on, which trips house power breakers, making use almost impossible. Farting noises and sweeping whistles from speakers, when amplifier is switched off. HF signal continues to pass when output is muted. Binding posts deeply recessed between heat-exchanger fins. Binding posts that are too slippery to be tightened, and/or undo themselves despite attempted hard tightening. Sharp, un-radiused metal case corners. Protection by a specially fast fuse with no spare(s) supplied. Fuse fails at worst moment through old age. Manual intervention required to reset an amp – after just a momentary power down, or the briefest short at one output.

9.2 Howlers * * * * * *

Under-PCB insulating material that traps moisture despite hours of drying out in warm air flow. Important for outdoor use and even for parties. Thermal grease (especially anything silicone based) oozing around mounted parts, or smeared about. Unfiltered fans blowing into electronic areas; and un-louvred or shielded air vents, allowing dust and filth to accumulate on PCBs. Fans switch off when the amplifier gets too hot. A fault on one channel stops the other channel or the whole amplifier working, without specific indication. Confusing, complicated or illogical LEDs (‘a green LED, marked ‘OIC’ flashes dimmer when headroom is exceeded’) or unindicated events (‘The power LED will continue to glow even though the amplifier has shut down’).

Poor Q-A * * * * *

Supplied ‘dead on arrival’. Something metal rattles inside the case. Uneven torquing-up of power device screws. Un-alignable lid screw holes. Random input polarity i.e. hot sometimes (XLR) pin 2, other times pin 3.

‘Unroadworthy’ construction * * * * * * * * * * *

Unlocked screws. Unsealed pots in an unsealed enclosure. Unpolarised connectors. Unretained, non-latching connectors. Unmarked connectors adjacent to others having identical pinout. Power devices, or audio or mains connectors or other wearable parts, hidden under an hour of metalwork dis-assembly. Sub-standard 4mm binding posts with short and/or slack receptacles Sub-standard 4mm binding posts with a slippery, ungrippable body. Flimsily mounted transformers that wrench free in transit, smashing other innards. Amplifier innards explode when outputs momentarily shorted. Amplifier emits smoke or fails when long speaker cable connected.

A small, worldwide amplifier interface quality catastrophe A type of 4mm output binding post made in Japan and regrettably fitted to thousands of ‘competitive’ professional and perhaps millions of ‘mid-priced’ Hi-Fi amplifiers, mainly made in Japan and the USA, is diametrically at variance with real needs. To be of any use, the hexagon shaped posts, moulded from a slippery plastic that is

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Choice, application installation and set-up certainly ungraspable if the grasper is sweating from any exertion, require an end-on hex driving tool, that is never seen or supplied. Second, the 4mm plug receptacle is foreshortened and has slack tolerances, so even the best sprung plugs are not gripped uniformly tightly, if at all. Third, the offending type is drilled to accept only thin wires, usually not above 0.75mm2. Astute purchasers will insist on having these illconceived posts sent for non-audio metal recycling, and replaced by higher quality parts, which are hardly expensive. Cliff and HCK are the foremost makers. In the following sections, first use, installation and sonic optimisation are methodically considered. Figure 9.3 BGW’s PS2, a compact, solidly-packaged yet budget professional amplifier, showing (centre foreground) four of the sub-standard output ‘binding posts’ blindly employed by pro and domestic makers in the US and Japan. Note input jack sockets on the right, which should never be used for fixed installation. The remainder of this design gains high marks for ease of servicing and inspection. Most of the air exits at the sides – a type ‘C’ airflow. © BGW

9.3 AC mains voltage New amplifiers should be factory set to the nominal mains (line) voltage for your country, and this voltage should be stated on the rear panel. Across the world, there are many combinations of voltages and frequencies. Five AC mains voltage ranges cover most Western and many Pacific rim countries: 1. UK, Australia, New Zealand, Malaysia: 240v Note 1: Since 1.1.95 UK is technically 230v +10%, −6% by EU law but in practice 240v +/−6% is maintained. In the year 2001, AC voltage throughout the EU will be in theory standardized at 230v +/−10% but the UK power authorities will nonetheless maintain 240v + 6, −10%. Likewise, continental European 220v will not increase to 230v in the short run, in many territories. Note 2: Western Australia ranges 240 to 260v.

2. Western Europe (continental) 3. Singapore, Korea 4. USA, Canada, Mexico, Taiwan Note 3: With regional and local variations from 110 to 120v. 5. Japan Note 4: With regional and local variations from 95v up to 110v.

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220v or 230v 220v 115v 100v

9.3 AC mains voltage Line frequency Frequency, always either 50 or 60Hz, is usually of no concern for operability. But when 60Hz transformers are used in 50Hz territories, the 20% change down in frequency increases transformer losses. Tightly designed types may prove unsafe or unreliable in professional use, where the amplifier is working hard in a confined space; or too noisy in domestic use. The temperature rise may be ameliorated by extra cooling. At worst, problems can be overcome by having a purpose-made 50Hz transformer fitted.

Tap adjustment The AC operating voltage of well designed power amplifiers made for international use can be adjusted (after disconnecting the supply) by removing the cover(s), and changing the transformer’s voltage tapping, identified by labeling, colour coding and/or by diagrams in the manual. A minority of highly specified amplifiers (e.g. BSS Audio’s EPC series) have automatic tap setting at switch on. The amplifier will set up for the mains voltage at this juncture. This type should not be ‘brought up gently’ using a Variac.

Headroom preservation The significance of tap setting is that as the majority of power amplifiers’ supply rails are unregulated, the rated or stated output power will only be developed if the mains voltage is closely matched to the transformer. Otherwise, continental Europeans get slightly fewer watts for their money from a UK made amplifier (without a 220v or 230v adjustment), since the continental 220 or 230 volt supply won’t allow amplifiers designed for 240 volts to develop their full, rated power. Fortunately the fractional dB loss in maximum SPL will be barely noticeable − but remember the same dB loss is also part of a potentially cumulative loss of headroom, which correct tap setting can restore without cost or compromise. The fraction of a dB could make all the difference as to whether a speaker is burnt out by accidental clipping.

Operable range Mains supply voltages are nominal. A ‘solid’ amplifier will be tolerant of wide voltage fluctuations that can occur in practice, and will continue to operate normally even if the incoming supply varies widely. In the outback and rural areas of some countries, and even cities in some places e.g. in SE Asia, mains supply voltages can surge to well above the nominal. Over 310v has been measured on a nominal 240v supply, both in Thailand and Malaysia. In many amplifiers, the driver and output transistors are not (and perhaps cannot be) rated to meet this level of excess. The surest solution may be to reduce the voltage permanently. In the absence of a 280v tap, a suitable, communal step-down autotransformer may be used; or as a temporary solution, a padlocked Variac.

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Choice, application installation and set-up

9.3.1 Safety earthing It is the norm in countries having high electrical safety standards, for substantially metal-cased equipment (or ideally, equipment with any exposed metal) operating from the AC mains, to have its casing bonded to earth, via a third wire, called the earth (or ‘earth-ground’ in the US; or CPC, the Circuit Protective Conductor in the British BS 7671 wiring safety standard). In a flexible mains cable (cord), the earth wire is coloured green/yellow by international convention, but in the USA and Japan, green is still used.

Raison d’être Safety earthing is required to prevent or forestall the chance of electric shock, the former caused by any accessible metal or other conductive parts of an amplifier becoming live, through (for example) internal damage, or a loose wire or part. With safety earthing, and assuming good connections, and a heavy enough gauge, the voltage on the exposed ‘en-livened’ metalwork won’t rise more than a few volts above ground, and fuses should immediately blow, alerting someone to the problem.

Exceptions Mass produced consumer-grade goods with predominantly plastic enclosures, and/ or where the mains connected components are doubly insulated, do not require a safety earth connection. The mains cable has just two wires. This method has its advantages, particularly if the audio connections are unbalanced. However, all but the smallest and most efficient amplifiers are sufficiently heavy to be unsafe to encase in the affordable but brittle plastics that are normally used. Shielding may also be questionable. Amplifier cases made with glass loaded plastic are feasible, but would be more expensive and use more space, than the steel, aluminium, copper and related metals still used in 99% of high performance amplifiers − professional or domestic. Plastic cases are also rarely shielded other than with conductive paint or skimpy stick-on metal foil. More solid metal shielding would defeat the cost saving.

Side effects Safety earthing is rarely electrically clean. Worse, it can readily form a loop in unbalanced systems, where the casework and signal common are one and the same, or are not wholly isolatable. One half of the loop is formed from ‘low’ side (usually the shield) of the signal connections. This ‘earth loop’ operates like a shorted turn, and high 50 or 60Hz and harmonic currents are induced, hence the names ‘hum loop’ for the condition, and ‘earth buzz’ for the sonic effect. Temporarily disconnecting the mains earth wire might prove the point, and is sadly practised by desperate or uneducated operators. But left off, it can put lives and property at risk. Removing the safety ground is dangerous, may invalidate all kinds of insurance, and should NEVER be practised. For hum solutions, look elsewhere (Section 3.3).

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9.3 AC mains voltage Ground solutions Power amplifiers with correctly wired, balanced inputs are literally outside of the (earth) loop. In the event of suspected earth buzz, the input cable shield connection to chassis should be lifted – via a high-wattage-rated resistor or better, small (0.1µF) capacitor; or opened. Some professional power amps have internal, hence tamperproof headers or switches that allow several options to be tried.

Groundlifting If inputs are unbalanced, the signal ground should be isolatable from chassis and mains earth. This is the purpose of a ground-lift switch, or linking bar that can break the connection between the case or chassis (earth) to the signal ground (0v), on a power amplifier. If these facilities are absent, the internal grounding may be modified by a practitioner. Groundlifting control may be accomplished with a resistor, an RC Zobel or even shunt network, or with back-to-back rectifier diodes. Unlike the former three, the latter provides high isolation until the difference in ground potentials exceeds about +/−0.6v, whereupon the diodes act almost like a hard connection.

Sound ground In large systems, and wherever there is RFI ‘smog’, the performance of power amplifiers and all other audio electronics can be improved by establishing a low impedance connection with the real ground mass (soil, clay, sand, soft rock and water table) connected to audio system safety earthing (mains earth). To be useful, audio earthing stakes must be away from others, as well as from buried power cables. If there are no accurate records of underground services, a power cable tracer should be used to select a location that is also safe to dig. The stake(s) must make a low resistance connection to the ground mass. To do this, the water table should be reached, which may be at least 4'/1.2m down, and possibly up to 25'/8m down. Electrical earthing rods are usually made of copper coated steel. Driving is made possible by the screw-together construction, like drain rods. The lowest, most RF-effective earth resistance is usually attained by driving-in several stakes over an area, or by excavating and burying earth mats, rods or wire grids. Water with a small amount of salt may be used to improve conductivity in temporary situations, but at the cost of rotting the copper and any exposed steel, as well as polluting the soil for many years. Urine has just enough salts and is the perfect eco-answer. Misuse of a technical earth-ground may prejudice safety. Use may require special dispensation by the authorities. Employ a qualified practitioner.

RCCB backup If for some reason it is impossible to get hum free results without breaking the mains safety earth connection to the amplifier casing, be doubly sure that a ‘Residual Current Circuit Breaker (RCCB), alias ‘safety power breaker’, rated to trip

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Choice, application installation and set-up at an imbalance of not more than 30mA, is used. If this trips out, it is trying to tell you that a potentially lethal current is flowing. Diagnosis is by unplugging everything then replugging one at a time. Often - but not always - the cause is the accumulation of small, individually harmless leakages spread across many items of gear, caused by capacitors to earth, in RF filters. This can be demonstrated if the equipment that appears to trip the RCCB is different after replugging in a different order. RCCB sensitivities vary also, between some 10 and 25mA for a maximum nominal of 30mA. The solution is to split the system between more RCCBs, each rated at 30mA. Don’t be tempted to uprate to a 100mA RCCB, as it will not offer enough protection against shock. Whenever power amplifiers are used outdoors, even if the only wetness for months in some regions is dew, and whether AC power is mains, or generated on site, an RCCB is strongly recommended for added protection against a 240v AC shock, which is far more likely to prove fatal in moist conditions.

9.3.2 Mains cabling For the best performance in given circumstances, it’s important that power amplifiers’ peak current draw is not impeded by inadequate power cables. For, excepting the present minority of amplifiers with power factor correction (see next section), peak current draw can be far higher than the amplifier’s nominal rated mains consumption in watts or VA, suggests.

Peak consideration It is particularly important to consider the peak voltage drop when multiple arrays of amplifiers are in use, and/or when the incoming mains supply is more than a few yards (metres) from the amplifiers. It is no less important to consider it in domestic and studio monitoring systems, even if the nominal amplifier power ratings are not especially high. For irrespective of rated power, the more the source impedance of an amplifier’s conventional capacitorsmoothed power supply is reduced (as is done in many high-end power amplifiers and installations), the better it can sound, but also the more peak current it wants to draw. It would draw an infinite current for a moment, if it could ! A sign of low supply impedance is that the reservoir capacitors’ recharging is bunched into ‘more amperes in less time’. Well recorded or live, uncompressed percussive music is especially testing of power supply recharge capability. Using a particularly realistic percussion recording, domestic high-end researcher Russ Andrews cites a power amplifier with a modest 200VA rated transformer (enough for a nominal continuous output of at best 150 watts) where a peak mains current draw over 100 Amperes for 1 millisecond was recorded, on a low impedance mains supply wired with non-inductive (‘fast’) mains cabling, and where line fuses were replaced by non-intrusive protection.

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9.3 AC mains voltage Mains impedance Irrespective of the power factor condition, the size (csa) of cable required depends on the cable length and the highest peak voltage drop that can be accepted. But the size (csa) of cable that it is worth using depends first on the mains supply impedance. As the mains impedance is finite and rarely as low as imagined, there are limits to how much employing low resistance conductors can assist sonic performance. A 100 Amp rated supply, for example, should maintain the nominal supply with at most a 6% voltage drop at rated rms current. In turn, this implies a maximum source impedance (at 50 or 60Hz) of 144mΩ (0.144 ohms). A good starting point is to measure the mains supply impedance. To do this: (i) Read the mains AC voltage with a true rms voltmeter, at the incoming supply or a socket adjacent to it. (ii) Decide on a known, resistive load (a heater is best) to be connected to the supply. Read the rating and voltage, e.g. 3kW, 240v. (iii) Calculate the design resistance of the heater element, e.g. V2/W ={ 2402 / 3000w } = 19.2Ω (iv) Observe the mains voltage fluctuation and if necessary, wait until the mains voltage is stable. Then connect (or switch in) the load, and observe the voltage drop. Note this. If there is any doubt about fluctuation, repeat the test: Stable, read off, then on, then off again. (v) The impedance is then calculated as follows. If the rms voltage was 243.0v and reduces 2.7v to 240.3v when loaded by the test load, the supply impedance at 50Hz is V/R = { 2.7 / 19.2 } = 140mΩ This would be about the maximum for a 100 Ampere rated, 240v supply, where the maximum legal drop is –6%.

Remedies If you calculate that the supply impedance is enough to take the supply voltage below its legal tolerance limits at its rated official load, then consult the supply authority. There is likely a bad connection in their system. The public supplier must make the voltage at your premises fall within the legal tolerances at rated load, and this is tantamount to reducing the impedance. If the impedance is within limits but a lower impedance is desired, ask your electricity supplier about the cost and availability of a higher rated supply, e.g. 500 Amperes. This will cost more to rent but the source impedance will be reduced by a useful factor.

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Choice, application installation and set-up Generator sonic limitations If the power source (usually for outdoor events) is a typical site generator, even rated at 100kVA or more, the source impedance is likely not especially low, and rises with loading, as ‘reactors’ giving a choke-effect, protect the windings. Therefore source impedance should be measured using a substantial resistive load, e.g. at least 4kW In any event, this largely prevents cables having lower resistance than that indicated by the rms current draw, from having any benefit.

Cable rating Once the supply impedance has been established, the connecting cable resistance can be computed. Based on not adding appreciably to the mains’ finite impedance, it can usefully have a resistance that is down to a quarter of the supply’s impedance. For example, the cable between the supply and the amplifier is worth reducing to about 35mΩ, if the supply impedance measures 140mΩ (say). This ratio might be taken further (e.g. to a tenth) in high-end domestic installations. The table below shows the DC resistance for some common cable sizes x lengths. Note that due to skin effect, the effective resistance at 50/60Hz is slightly higher.

POWER CABLE RESISTANCE – loop value Length

2.5mm2

4.0 mm2

6mm2

10mm2

1m/3' 5m/16' 10m/32'

13 mΩ 65 mΩ 130 mΩ

8.5 mΩ 42.5 mΩ 85 mΩ

5.6 mΩ 28 mΩ 56 mΩ

3.5 mΩ 17.5mΩ 35 mΩ

9.3.3 Power factor correction The peak current draw or demand just described is known to power engineers as a ‘bad power factor’ − in part because the network capacity is being under, then overutilised. It is pertinent to note that peak current draw is markedly reduced in all cases if the power supply is an HF (switching) type with power factor correction (PFC). In EU countries, EMC legislation is (since Jan 1996) setting compulsory, and increasingly tight, limits on the peak current draw of most mains powered equipment sold in the EC. In legislation that is forthcoming, PFC will be mandatory for all new audio power amplifier designs rated above a few hundred watts overall. As power factor corrected current draw is purely sinusoidal (varying in phase with the mains voltage waveform, as a resistive load behaves), mains cables can be thinner, and if long, simply and conservatively specified for a resistance that yields a reasonable voltage drop (say 4v peak re. 240v rms) at the nominal, maximum rms current draw cited by the maker, with pink noise drive. In touring PA, power factor correction will close the sonic difference between hearing one small section of a large system (sounds good) and hearing the whole system (louder, but never sounds quite so good). Without it, the interaction, and crosstalk

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9.3 AC mains voltage through the mains supply, of many power amplifiers all connected to a common main riser or generator with typical supply source impedances, can be immense. In theory, PFC could be ‘bolted onto’ an existing multi-kW rig but this would be percieved as prohibitively expensive, weighty and bulky. Its use en masse will have to wait for overall re-equipping with power amplifiers incorporating PFC. This is a far less expensive and radical change for those amplifiers already employing HF switching power supplies. Figure 9.4 Crest Audio’s goliath model 10001 cannot be safely lifted by most individuals without aid ! It contains four channels of class G, each capable of over 1kW of power delivery, and as such, is relatively light and compact. The total potential continuous mains power draw, particularly on 100/115v supplies, is so great, in excess of standard power outlet ratings, that individual hosepipe-sized power cords are fitted per pair of channels hence the two power switch/breakers seen here. Notice the central tunnel cooling, and the general symmetry of the weight disposition, with end-on toroids up front, and reservoir capacitors behind. © Crest Audio Inc.

9.3.4 Mains fuses and breakers Fuses are purposefully weak links that will burn away safely and cut further current flow. In most power amplifiers, a cartridge fuse or fuses either located on the front or (better) on the rear panel or inside, protect(s) the amplifier and the building from fire, in the event of a major fault, or short from live to the casing. When installing or servicing, check the tightness of the fuseholder cap, if screw fitting. If installing a second-hand amplifier, it is worth checking the fuse in situ is the type specified in the manual, or close. If the value or speed rating are higher, the fuse(s) may fail to protect on the rare occasion when needed. Insurance may be invalidated. If fuses are tarnished, they may be best cleaned with a super-fine abrasive, then degreased with Hurd’s No.1 Grimewash comprising 5% by volume of household ammonia in isopropyl alcohol; or even better, a proprietary and qualified electrical contact enhancer, e.g. from Caig Labs, USA. Fuses age, and with time, can fail at any time. As with lightbulbs, weakened ones usually expired at switch on, when the surge current heats and stretches them most. You could avoid this by changing all fuses annually. But this would be a waste. For near uninterrupted use, either keep spares at all times, or fit a circuit breaker.

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Choice, application installation and set-up If the mains fuse has blown or blows repeatedly, the amplifier is being misused, is wrongly set up (are you really sure ‘there is no mains volts setting’?), or is faulty. To save blowing expensive, scarce or hard-to-access fuses each time the amp is powered up after the fault is believed to have been cleared, professional troubleshooters employ a simple jig which places a low to medium wattage mains lightbulb in line with the AC supply. If the fault persists, the lamp will light and the fuse will be saved. Do not expect to pass a high level signal into a load in this condition. Ideally only ever replace a fuse with another of the same type and current rating. A temporary current uprating, to twice at most, is just passable so long as the correct fuse is sourced and installed ASAP. Mark the equipment: ‘Temporary Fuse! Date:_ /_ /_ Replace ASAP in 24 hours’. For more information, refer to chapter 6. Circuit breakers (thermal−magnetic and/or pneumatic ‘breakers’) are alternatives to fuses. In touring PA, common industrial types double as panel power switches, give effective protection, clear fault indication and few problems, and commonly last out the equipment life. If not fitted originally in a given amplifier, breakers may be installed externally in the amp rack, and the amp’s interior fuse value(s) may be increased (with due care) so the breaker trips out instead, at the correct current. Circuit breakers of the magnetic or pneumatic type are to be preferred in lieu of fuses for all high performance power amplifiers. First, they are solidly connectable by machine screws or soldering, unlike most fuses. Second, suitable types have a lower resistance than a like rated fuse, and also a relatively invariant resistance, with temperature and over time. The contact resistance of the average, miniature cartridge fuse when fitted in the low quality holders that are commonly used, is potentially high enough to swamp the benefits of a heavy-duty supply and cabling. Third, in conventional supplies (without PFC), fuses that are correctly rated to protect the power supply parts from a fire will be appreciably heating up on current peaks under hard drive conditions. The significance is not just fatigue but also modulation, since a cartridge fuse’s resistance will approximately double before the wire melts.

9.3.5 AC mains connectors, amplifier end In consideration of the high peak currents that can be drawn by high performance power amplifiers, and the fact that most connectors have (or can develop) as much resistance in their contacts as a metre or more of the cable they are on, superfluous mains connectors are best avoided, however convenient. If the AC mains cable you are fitting has to plug into the amplifier, at least make sure the fit is secure and tight. For those plugs where contacts are clamped by screws (cf. soldered), a regular tightening programme is required. Even if the screws do not loosen (due to the relaxation and creepage of the soft metals that are best for electrical contacts), vibration and/or thermal expansion cause ‘fretting’, where the microscopic clean contact areas are eventually exposed to the air, and/or over which oxide ‘crud’ is spread. This applies to all non-soldered contacts [4].

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9.3 AC mains voltage Professional operators will usually replace the socket with a fixed cable, secured by a gland. Well designed pro power amplifiers have accessible mains terminations and even optional panel plates, to facilitate such changes. If a connector must be used, whether on the rear of every amp, or on the rear of each amp rack, the order (for ruggedness and security of contact) is approximately as follows: ⇐Solid CEE,125A CEE,63A CEE,32A CEE,16A

Flaky⇒ EP3 20A* EP LNE#

IEC 16/20A

Neutrik Powercon Bulgin 15A IEC 10A XLR LNE # Bulgin 5A

IEC 6A

A = nominal rating in Amperes. # Officially not for domestic use. * Used in older professional systems. Not legal. Must not be used ! CEE = CEE-form type, round pin industrial connectors to BS EN 60309-2:1992, previously BS 4343. Figure 9.5 The IEC 6 Amp connector is unlatching, has quite loose contacts, and is readily broken. © Canford Audio.

Figure 9.6 The EP mains connector. It is rugged, compact and makes good contact, but is not legal for domestic use. © Canford Audio.

Figure 9.7 CEE-form AC mains connectors, a development of the 1930s British roundpin mains plug, available in ratings up to 125A rms, are the preferred choice for touring power distribution, and for advanced high-end amplifier installations, where the size and garish plastic are not a problem. © Canford Audio. Figure 9.8 Neutrik’s new Powercon AC power connector is based on their Speakon. Seen here are the blade connections on the Powercon receptacle inside an amplifier case. © Canford Audio.

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9.3.6 AC mains connectors, the power-end Where feasible, the best sonic performance is most likely to be achieved when the AC mains is ‘hard-wired’ throughout. In fixed installations and high-end domestic setups, this is possible (at a cost) using 60 Ampere or greater, metal-cased ‘industrial’ switch-fuse boxes (e.g. MEM), which contain heavy-duty, solid brass terminals with double screws. Failing this, seek the heaviest connector that maintains the lowest contact resistance in practical use. The UK’s 13A ‘square pin’ (actually rectangular) and pre-WW-II 15A and 5A ‘round-pin’ power plugs and sockets are the world’s chunkiest domestic AC mains connectors, and the better quality examples are increasingly used by Audiophiles and serious audio facilities throughout the world, where mains connector quality is mostly of a far lower order. Whatever connector is used, clean the contacts regularly. In fused plugs, clean the fuse contacts and fuse also. Caution: Copper cables ‘creep’, causing brass or other terminal screws to slacken in time. In mains connectors, this can ultimately cause arcing and fire. All mains conductor clamping screws must be regularly checked, shortly after installation, then annually at least.

9.3.7 Mains wiring practice, domestic and studio Note: In some countries, some of the recommended techniques may require special dispensation by the electrical supply or safety authorities.

Ring and Spur In many installations, but only in countries adopting the custom, the mains power for domestic and light commercial buildings is wired in a ring (Figure 9.9). This saves copper. The idea was originated in the UK in 1939, when the import of vital copper ore was for the second time in under 25 years at risk from Germany’s Uboats. However, AC power for high performance audio power amplifiers is best fed from a dedicated radial spur cable, taken directly from the incoming supply, where impedance is lowest (Figure 9.10).

Figure 9.9 The 30 Ampere diversity ring supply that is universal in the UK for domestic and light industrial premises. However, it is unsuited to powering audio equipment, especially power amplifiers. © Studio Sound

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9.3 AC mains voltage Figure 9.10 In a dedicated listening or monitoring installation, every power amplifier and all other major audio equipment is supplied by an individual spur noded at the incoming supply. © Studio Sound

Star system This technique of running individual cables to equipment from the lowest impedance, nodal point is analogous to ‘star earthing’ and virtual earth mixing. Like these, it helps each source reject external influences, and reduces the sources’ own influence on all the other circuits. This means: (i) sag (including music-modulated stuff) caused by the amplifier(s) and experienced by equipment on other circuits will be attenuated, and (ii) the amplifiers’ reception of interference from noisy appliances on other circuits will be attenuated, and these noises quite possibly will be done away with altogether.

Rejection Typically, a spur’s interference-cum-disturbance attenuation ranges from a modest (but still welcome) −8dB, up to −40dB at audio (10Hz-20kHz), and more at radio frequencies [5]. By contrast, ordinary mains interference suppression filters are predominantly focused on RF: they provide no protection below 200kHz, and only come in strongly above, in the MHz realms.

Sonics For these reasons, radial spurs are the norm for the technical, audio equipment supply in most competently wired studios and dedicated listening rooms. If you have mains problems, it pays to check, particularly in installations where despite using all the right equipment, the sonic quality is never quite right or changes daily.

Installation example Begin by measuring the mains supply impedance (section 9.3.2), remembering that the amplifiers’ power cabling will be based on this. If not considered low enough, a higher current supply may be installed; this may require special dispensation from the supply authority − as you will not be buying more electricity commensurate with the enlarged supply capability.

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Choice, application installation and set-up With a multi-amped monitoring system, powering the amplifiers from individual, exclusive spur cables, one per amplifier, is worthwhile for the closest approach to a clean supply. Ideal cable size is determined according to distance, and the mains impedance: see the discussion and table in section 9.3.2. In installations where the power cabling is to be buried in the structure, it’s best to err on the side of generosity with the cable size, as the mains impedance might be reduced, and even 10mm2 conductors won’t incur a great deal of added expense alongside the cost of typical building work. Low inductance cables improve sonics and reduce 50/60Hz fields and harmonic radiation, but types made for speaker wiring may not be safety approved for permanent wiring use in some territories. In lieu of using purpose-made low inductance cables, place Live and Neutral (return) conductors in ‘close parallel’ configuration, so they are adjacent throughout the run, i.e. no centre earth conductor. This is achieved easily enough with single conductors. As the conductors are likely overrated for current, the proximity should not normally require the circuit’s current rating to be derated. But note that close send/return placement is also subject to proximity effect, which causes a dynamic resistance increase at high currents. When planning a new room or a revamp, if the mains supply impedance is low enough for it to be worthwhile, it’s useful to organise the house’s incoming supply and the amplifier placement to be adjacent, for the shortest power cable run. This may mean moving the supply or the amplifier placement, or both. Even in domestic installations there is a great deal to be said for having amplifiers in a room that is separate from the music, to lessen the effects of vibration on sonic quality. This ‘other room’ might be also where the house mains supply enters. At the same time, the length of the speaker cables must also be considered. The location and positioning of the speakers will almost certainly have to take precedence, but if a room is created afresh, the incoming power and the amplifier room (if one) would be best placed behind the wall(s) nearest the speakers.

Mains ecology Dedicated low impedance power connections to power amplifiers resolve (or at least greatly reduce) the interference problems within the building. Outside is another matter. Most AC power has a high harmonic content (from 5 to 20% harmonic distortion). On a scope, it appears flat-topped (Figure 9.11). In those mainly urban and industrial areas where the incoming AC power is excessively ‘brown’, interference ridden, or has erratic voltage and harmonic fluctuations, and if the supply authorities are unable to improve matters, you will need to either install a power conditioning plant which may range up to a dedicated substation, or else generate clean AC on site [6]. This will involve common-mode chokes which must be rated at high currents commensurate with the supply’s nominal capability. This is for the same reason of not adding unnecessarily to source impedance, as well as not having the choke’s core saturate, so its value abruptly changes.

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9.3 AC mains voltage

TYPICAL AC MAINS SUPPLY (i) Ideal sinusoidal wave. (ii) Flat topped, polluted condition. (iii) Current draw centered on Vpeak.

(iii) (ii) (i)

Figure 9.11 If the mains were only loaded with lightbulbs and heaters, the voltage would be a pure sinewave (i). Today, the supply voltage is nearly always flat-topped (and flat bottomed) (ii) which implies high harmonic content. The flat top/bottom is caused by thousands of audio and other capacitatively smoothed supplies drawing current in a peak (iii) only about the peak voltage point.

Failing this, AC mains pollution commonly varies throughout the day and the days of the week. Outside of heavy industrial areas where power-polluting processes (e.g. steelworks) operate every day around-the-clock, most listeners find that the cleanest power and best sonic quality is experienced after midnight, through to the early hours; and at weekends. Peak voltage is also higher.

Conclusions All power amplifiers affect and are affected by the mains supply. Class A amplifiers are least affected, but can still cause disruption to other equipment sharing the supply. By increasing the potential for high peak currents, dedicated, low resistance spurs accelerate the recharge of class B, A-B, G and H’s amplifier’s reservoir capacitors after power-hungry transients, and class A amplifier’s reservoir capacitors after every dip in the mains supply. A limited number of modern amplifiers have regulated supplies. They’re largely immune to performance variations caused by supply voltage and harmonic content (seen in the non-sinusoidal waveshape) [7], and while this isn’t automatically conducive to good sonics, it helps a great deal, the more the mains supply conditions are bad. Overall, the benefits of a low impedance, spur or other diversified, cleaner supply are clearly evident to the ear at higher playback levels on even modest setups, but are all the more apparent in high performance systems.

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9.4 Input connections i) Phono, unbalanced (RCA connector) With domestic power amplifiers, even high-end models where makers should know better, input connections are still almost universally based on ‘Phono’ plugs and sockets. They were originally developed by RCA (Radio Corp. of America) as an early constant-impedance coaxial connector, for VHF. Audio use was through historic accident. Quality and price ranges enormously. For a list of reasons, phonos are frankly unsuited to high performance audio. In the past decade, high-end makers and accessory suppliers have worked on solving or ameliorating some of the defects of the Phono plug and socket, namely the plug’s poor cable support and the need to solder down a hollow tube; the way in which the live contact (when plugging-in) is made before the earthy side (causing loud bursts); the tendency of the inner and outer contacts on both plugs and sockets to slacken; and sockets’ isolation from the chassis − to mention a few. On the plus side, metal bodied Phono connectors can do a good job of shielding the inner connector at RF. As they have only two connections, Phono connectors are usually limited to connecting unbalanced signals, as follows: (i) Outer to shield. (ii) Inner to signal hot. On a few amplifiers, there is a balanced input via phono sockets, which requires connection as follows: (i) Outer to signal cold (note socket body is isolated from chassis). (ii) Inner to signal hot. (iii) Shield soldered to fly-wire. Connect to chassis. It does not matter whether the source is balanced or unbalanced, but if the latter, do not connect the shield at source end. Figure 9.12 An improved phono connector. © Canford Audio.

ii) XLR, balanced (‘Cannon’) In high performance, professional power amplifiers, XLR connectors are almost universally used, usually the 3 pin type. Whilst not perfect, they have proven to be intrinsically rugged and reliable. The XLR system, originated by Cannon (US) in the early 1960s, comprises matching, latchable male and females in both line (cable) and chassis mount ‘plugs’ and ‘sockets’. XLRs are also used by some high-end domestic equipment makers, usually when the equipment has balanced connections. But some of these makers appear isolationist and ignorant of their professional colleagues’ long established pin connection and gender conventions.

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9.4 Input connections Figure 9.13 XLR connections

See section 9.4.2

Female XLRs (sockets or receptacles) are almost universally used as the chassis mounted input on power amplifiers. There are at least two ways of wiring the incoming male cable plug, assuming one is used per channel. For operation from a balanced source, a 3 pin XLR plug is wired as follows, using 2 core (1 pair) shielded cable: (Figure 9.13, i) Pin 1 – Ground or shield Pin 2 – Hot or + Pin 3 – Cold or – Today, this is an international convention (e.g. AES14-1992, and within IEC.268). It was however first adopted in the ’60s by (for example) RCA in the US and the BBC in the UK. Yet many makers in the USA have been slow to revert from an alternative standard that developed, where the polarity is reversed, i.e. Pin 2 = cold and Pin 3 = hot. This can cause problems with polarity ‘matching’ when equipment wired to opposite conventions is used together. See section 9.4.1, Balanced Polarity and Shielding. This section also considers shield connection options.

Unbalanced XLR For short cable runs (ideally under 10'/3m) from unbalanced sources, ideally only in a rural, domestic environment, unbalanced connections at the amplifier end may be satisfactory. This arrangement requires only single core shielded cable, but has no hum and noise rejection, so it should be rejected unless it proves satisfactory. The following connection (Figure 9.13, (ii)) applies to (i) using a balanced input from an unbalanced source with an unbalanced connection, and (ii) in a wholly unbalanced system. Pin 1 – Cable screen/signal return Pin 2 – Live input (Hot or +) Pin 3 – Link to pin 1 (if input balanced) or leave open (if unbalanced).

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Figure 9.14 Neutrik’s XLR connector, the world standard for professional line connections, and many professional speaker connections. © Canford Audio.

iv) Jacks, 1/4", balanced and unbalanced (Phone Jacks) Mono (2 pole) and ‘A’ gauge (‘stereo’ or 3 pole) jack sockets are used on low quality and ‘contractor’ equipment. Originated by the British Post Office nearly a century ago as telephone jacks, they make reliable contacts when regularly inserted and withdrawn, but not when installed and left. For this reason, jack connectors should never be used for fixed, high performance audio. The sole exception is in systems that are moved and replugged almost daily, or where labour is available to polish the connectors weekly − using a proprietary micro-abrasive or other suitable cleaner. Otherwise, in equipment having solely jack connectors, or requiring jack connectors for practical application, the connectors are best changed. The balanced connection is like XLRs, where: Sleeve = pin 1 = shield Ring = pin 3 = cold (–) Tip = pin 2 = hot (+) See also section 9.4.1, Balanced Polarity and Shielding.

v) DIN DIN connectors, dreamt-up in an austere post-WW2 Germany, are the most fiddly connectors to wire, are wholly unsuited to normal wire gauges (anything fatter than 34 swg), and the cable clamping and assembly quality is commonly mediocre to poor. Pin contact can however be surprisingly reliable. The tinned contacts do not look pretty, but sonics can be good. While most commonly encountered in the 3 or 5 pin 180° format, there are versions with 4, 6 or more pins, and some types span 240°. While rarely found on the inputs of few (if any) modern high performance power amplifiers, they have played a part in several classic designs. In many instances where power amplifiers are designed as part of bespoke, esoteric or early systems approaches, DINs have been used to carry DC power, control signals, etc., sometimes along with line level audio. Higher quality types made by Preh have robust latching. The DIN wiring standard is prolific, but also idiosyncratic and outdated, e.g. there is nothing for balanced power amp inputs. The central pin (on 180° plugs) is always 0v (shield) and is always numbered (as pin) 2, regardless of the number of and numbering of, the surrounding pins.

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9.4 Input connections vi) Terminal strips (barrier strips, screw terminals) These are found on the back of low budget amplifiers intended for (or as an option for) fixed installation. They can give good results provided the metal is substantial enough to allow positive screw tightening without fatigue or thread stripping, and provided the metal and plating are both appropriate to high quality sound. The correct eyelet or ring terminal must also be used on the wire, which must be either soldered, or crimped correctly with the proper tool. Support of the delicate cable conductors is usually nil, and may be difficult to arrange. And the nuisance of disconnection can create a psychological barrier to regular amplifier inspection. If connections are balanced, see also section 9.4.1, Balanced Polarity and Shielding.

vii) Proprietary input connectors In both high-end power amps, and professional touring amplifier racks, unconventional input connectors may be used for good reasons. Mostly these are shielded, multipin types, often with a latching interface, specially plated, with specially low resistance contacts. Most are made for arduous use in professional, industrial, military or aerospace equipment. Examples include Camac, Lemo and Lytton. Wiring conventions are naturally ad-hoc; for connections consult the amplifier or rack maker.

9.4.1 Balanced polarity and shielding With balanced connections, signal polarity can be one of two ways. With amplifiers, variations apply mostly (but not always just to) inputs. It is not unknown for the inputs on samples of a given model of amplifier (one intended for use in the US say; or previously rewired under the hood when hired to work with other gear) to be wired differently from today’s ‘Pin 2 hot’ norm. Random or indeterminate signal polarity can cause anything but the simplest loudspeaker system to perform below its potential. Some highly specified professional power amplifiers have featured PCB links, headers or other internal switches allowing XLR (or other balanced connector) polarity to be set for ‘pin 2’ or 3 ‘hot’ (or equivalent thereof). It is worth noting that if all the connections are balanced, then if every amplifier in a given system is wired for the ‘wrong’ polarity, then only the absolute polarity of all outputs is flipped. This would have a relatively subtle effect on sonics, and could be simply corrected by flipping the polarity of the inputs to the active crossover. Otherwise, if the balanced (XLR or whatever) input sockets in the amplifiers in a system have opposing hot and cold conventions, system polarity should be made ‘all one way’ by rewiring the amplifiers’ sockets internally. In large systems, nothing should ever be taken for granted: one should double confirm polarity with a tester. This way, amplifiers that have been carelessly serviced, so the polarity is flipped inside by wires being replaced wrongly, show up. When power amplifiers or sources with non-standard polarity wiring cause polarity errors in a pro-touring system, temporary ‘phase-changer’ tails (short leads) may be employed as a fix. Ordinary XLR-to-XLR signal cables should never be cross-wired.

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Choice, application installation and set-up The optimum connection of the shield in a balanced system is less fussy than in unbalanced arrangements, yet there are options and permutations that require consideration and often, practical evaluation. The starting point for shield connections in all-balanced connections is simply tying it to ground at each end. In practice, this usually means the chassis, or whatever else the mains safety earth wire (CPC) is connected to. The problem is that this almost inevitably creates a loop down which surprisingly high currents can flow, which can cause or aggravate hum in other parts of the system, if not directly. In the following choices, it is assumed that pin 1 at each end is not connected to 0v (it is the wrong place to connect a primary EMI shield to), but solely to chassis/earthground, as just discussed. The shield connection options are: (i) Connect the shield to pin 1 at amplifier input only, as illustrated in Figure 9.13 (i). This connection, uniquely important to power amplifiers, ensures the shield is tied to ground as long as the cable is plugged into an amplifier’s input. Without this, there is a risk that damage might be caused to the amplifier and/or speakers. (ii) Connect shield instead to pin 1 at the source end only. This system seems no different on the surface, but it offers superior at RF noise rejection. This ‘tie back the shield’ connection cancels the effect of conductor-to-shield capacitance imbalance, which otherwise reduces CMR at RF, where it is most needed. It is thus the optimal connection for most audio connections, where the previous consideration does not apply. But this connection is risky as it stands for amplifier connection in large systems, as by just dislodging the returns multicore connector at the mix position end, n kW of amplifiers would be left dangling with 100m or so of unshielded cable, and in the words of Steve Dove, would be likely ‘Screaming off into the ionosphere’. (iii) Connect as (ii), but also connect as (i), but via a small capacitor. This may be fitted between pin 1 and chassis inside each amplifier. This has no appreciable effect at audio frequencies, but ties the shield increasingly to ground (chassis) at RF, at the amplifier end, safeguard if the other end is disconnected.

9.4.2 Quasi balanced (unbal-to-bal) Even if the source equipment doesn’t have balanced outputs, a power amplifier with balanced inputs can still be usefully connected in Quasi-balanced configuration. This arrangement, while offering less protection than good overall balancing, can nonetheless greatly reduce hum and RFI pickup over simple unbalanced connections. Two core shielded cable is required. Alternatively, twisted unshielded conductors may be used, at the owners’ discretion. At the amp end, the connections are conventional, provided that pin 1 on the chassismounted socket is connected (or can be re-wired to connect) to chassis, instead of signal 0v. At the source end, ‘cold’ goes to the unbalanced output’s earthy ‘0v’ side. The shield is not normally connected at this end. See Figure 9.13, (iii).

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9.4 Input connections

9.4.3 Input cabling Input cabling to power amplifiers should have above all low and balanced capacitance (unless very short), low microphony, and good screening.

Capacitance Low capacitance avoids placing any unnecessary loading on the source equipment at HF − particularly when the cabling is long, as in PA applications. ‘Low’ means below (say) 100pF/metre. 600pF/metre would be considered high, and 200 to 300pF/m medium. The pF/ figures can be looked up in catalogues or measured. In balanced cables, the pF/ figure used should be 1.5x the capacitance between the cores, as the capacitance from each core to shield adds half again to the total. But if the cable shield is ‘tied back’ to the source end, the core-to-shield capacitances no longer appear as loading (at least in balanced systems), leaving only the core-to-core capacitance, as in twisted but unshielded cables.

Microphony Low microphony is achieved firstly, with a low equipment source impedance. If the source impedance is necessarily rather high (anything above 1kΩ), microphony can be reduced by suitable physical design that isolates or quickly damps or counteracts vibration between the cable’s insulators and conductors. It is an important consideration in systems where the load and (particularly) the source impedances are higher than the norm, usually when the source is valve equipment. It may be instructive to listen to a sound system, after suspending the amplifier’s input cabling (e.g. on string hangers) − so it’s no longer in direct contact with the room surfaces. In PA systems, return multicores are frequently already hung or at least slung, for practical reasons.

Shielding Cable shields have two jobs. They can totally reject electrostatic fields that might cause hum or signal breakthrough. They can also intercept RF, and depending on its field strength, at least partially drain it away. But note they have little or no improving effect on magnetically induced hums or buzzes. Good screening is achieved by choosing the right cable for the job, as well as by correct screen termination. Correct termination is always direct to chassis (via pin 1 if XLR) − at least at RF [8]. In unbalanced systems, this must be performed at the power amplifier end. In balanced systems, the connection can be either source or power amp end. If the chassis grounding of the shield conflicts with another grounding scheme, then connection may be made via a capacitor that maintains a very low to relatively low impedance across RF, e.g. 100nF, 400v rated, in parallel with say, 100pF. At RF, the best shielding is achieved with aluminium foil, or with multi-layered braided (knitted) shields [9]. The latter is expensive but may be needed if RF pollution levels dictate it. Single braided cables are next best. Good braided shields are highly resistant to damage by repeated coiling, twisting and trampling. Without this factor,

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Choice, application installation and set-up single twisted ‘lap’ shielding would also be rated, but random gaps develop or existing gaps enlarge as soon as it is just bent around a corner. This makes single lap shielding unsuitable for shielding high performance audio connections unless ambient RF levels are known to be low. The double lapped version is better, but still inferior to a good braid. Likewise, but far less readily, the foil in foil-shielded cables will tear apart if over-handled. While degrading RF shielding, gaps in a shield have little or no effect on intercepting hum and buzz frequencies.

Summary Foil- and lap-shielded cables are best kept for fixed professional and domestic installations, and internal wiring inside equipment that contains no RF, digital or switching psu circuitry. For touring PA, the best and longest-lived RF immunity will be had with braided and even conductive plastic shielded cables. In domestic installations with low levels of RF, and E-M fields (e.g. by spacious layout and/or use of low inductance mains and speaker cabling), unshielded cables may exceptionally be used, particularly if twisted or maximally mutually−coupled, and connected to a balanced input.

9.5 Output connections Power amp output connections are unlike most others in audio, in terms of the current magnitude (commonly peaking in the order of low tens of Amperes yet well over 100 Amperes in some cases), the peak voltages (potentially enough to cause shock), and the consequent size and weight of conductors and insulators that have to be accommodated. These stress the need for a stably low resistance, together with good enough insulation. Excessive safety legislation at the connector is pointless when the speaker cables could just as easily be abraded or bitten into. Now that audio amplifiers exist with output voltages equal to the highest single-phase mains voltages, it seems inevitable that sometime in the future, safety protection using some form of current-balance monitoring (as in an RCCB) will be required on the speaker circuit.

Contact care If unplugged when programme is present, current at the ‘moment’ of separation could be at any value between zero and many Amperes. Whenever a substantial current flow is abruptly ceased by breaking connections, inductance in the broken circuitry sets up a back EMF or ‘kickback voltage’ that is high enough to cause arcing. This draws out the contact period. Even if it only extends it for milliseconds, any arcing (just like welding) will inevitably vaporise some part of the connector surface. It will also create oxidation products that can corrode or corrupt the remaining good surface metal. It follows that in high performance audio systems,

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9.5 Output connections power amplifier output connections ideally shouldn’t ever be broken or remade when large signals are present. With small signals, typically below 1v peak, there is little current flow, so arcing doesn’t occur or has insufficient energy to more than heat the metal surface. Connection under these circumstances is safe. With some types of connector, e.g. 4mm, arcing will with usually occur at a location (in this case the tip of the plug and socket) that isn’t needed for conduction.

Practicalities If there is any choice in the connectors used in a system or installation, consider that misconnections are likely on adjacent amplifiers or channels which use input and output connectors which are either compatible (eg. unbalanced XLR-F in and XLRM out); or where there are converter leads in use. For example, the outcome of a Speakon to male XLR cable could be easily (in some large systems) plugged into an amplifier input, particularly if speaker cables are not always readily distinguishable from input cables.

i) 4mm binding posts (4mm terminals) Binding posts are amongst the oldest means of connecting electrical apparatus. Traditionally, bared wires were threaded through the hole in the post, then wrapped around one or more times, then clamped down tightly under the knob part. But unless the binding post can be torqued down tight enough to crush the wires solid, this method provides little area of gas−tight contact, or at least can leave a lot of conductor bare to the air, so sonic performance quickly decays with the oxidation and pollution of the conductors’ skin. In modern practice, the procedure has been simplified to threading the bared ends of speaker wires straight through the post, then clamping them down securely. This way, the post and conductive shoulder makes immediate gas tight contact with the single thickness of conductor − all that is needed. By crimping bootlace ferrules onto them, professionals may prevent conductor tails, if stranded, from getting frayed and scrunched-up. Modern binding posts also allow a 4mm plug (‘banana jack’) to be inserted from the rear. Alternatively, in the better designs, the conductor hole is 4mm (dia) or slightly larger, so both a 12mm2 conductor and a 4mm plug can be clamped down. This latter best suits the poorly-sprung plugs that most need clamping. High grade plugs with low contact resistance have ‘leaf’ sprung contacts on all sides, and localised hard clamping will damage them unless the torque is judged carefully. The binding posts should be turnable with ease with the fingers. Slippery plastics are out (see ‘Howlers’ section 9.2). Knurling, fluting or surface texturing are best. To be of any use, the commonly encountered hexagonal shape would require a suitable end-on driving tool − that is never seen. The best designs (Cliff in UK) give good access for threading fingers, and have plain, large screwdriver slots end-on, for tightening or merely checking tightness, when they are deeply recessed or otherwise

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Choice, application installation and set-up out of easy hand reach. They can even be tightened with a coin. No special tools are needed. Spacing is conventionally 19mm (once 3/4"), to allow double 4mm-jacks to be used. Traditional and modern binding posts as well as 4mm plugs and jacks, single and double, may not meet modern safety standards, e.g. LVD in EU countries, as bare conductors are usually exposed. This may signal their demise in some territories. Alternatively, added insulating shrouding, firmly clipped-on after wiring, may enable them to meet safety requirements. At the time of writing, some makers are fitting a removable, shrouding ‘end bung’. Perfection in the external details and utility of binding-posts is fruitless without suitable terminations behind the panel. These are nearly always solderable, but also threaded, allowing wires to be connected via eyelets or tags which can be securely nutted down. The eyelets may even convert to push-on type blades. These options mean that sockets can be changed without soldering. The significance is that 4mm binding posts are proud components and usually made from brittle brass, which is occasionally snapped or bent in transit, installation or professional tour rigging. So the need for an occasional replacement is to be expected. The benefit of not soldering is that more than one wire of more than one gauge should come together at output sockets; and that it is almost impossible to desolder heavy, PVC insulated wire without meltingback and damaging the insulation. Notice: Copper-based cables, particularly those with a small number of relatively thick strands ‘creep’, causing brass or other terminal screws to slacken in time. To arrest degradation in sonic quality, all connectors based on gas-tight conductor clamping must be regularly checked, shortly after installation, then annually at least.

ii) 4mm sockets Sockets (receptacles) are notionally ‘safer’ than binding posts, as live parts are inaccessible with plugs fully home, and are more deeply recessed when plugs are absent. Special high voltage types are available with an extra deep recess. But unless the plugs used with them have high voltage sprung shrouds, live terminals can still be exposed by partial withdrawal. Some types can offer switching when a plug is inserted. Contact rating is typically up to 32 Amperes for quality types. Connections may always be soldered. The better types have optional threading for nutted eyelets, or push-on Lucar blades. The preceding connectors (i) and (ii) are single pole. This gives ample flexibility, and the cost is low, but it increases the risk of wrong connections, and the time taken to make connections, in large ‘active’ and multi-amped systems. Multipole connectors with 4, 6, 8 or more pins are therefore de rigueur for the external rack connections to speakers, in touring systems.

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9.5 Output connections iii) XLR See prior section 9.4 for a general description. XLRs have long been employed as output connectors on professional amplifiers. But with modern safety legislation, and also increasing maximum output voltages, they are viewed as unsafe, because of the long established convention that ‘signal emerges from males’ (i.e. on pins). This is fine at line levels, but not at anything above 40v, the voltage at the output of a relatively modest amplifier. Some professional users, including the BBC, have overcome the problem, by simply reversing the gender on output connections, so the output is on a female receptacle. Another snag for use for output connections, is that few XLRs can comfortably manage the diameter of sheathing on 2 core PVC cables above 1.25mm2. XLR output connections are usually wired as follows: Pin 1 – Black or – Pin 2 – Red or + Pin 3 – not connected. XLRs were originally expensive. Today, thanks to the efforts of Neutrik (cf. the earlier US makers), utility has been enhanced and high overall quality maintained, while the real price has halved in the past decade. Completely plastic XLRs have been made available by other makers. These cost a little less, but most of the quality is lost, viz. EMI shielding, ruggedness and inter-plugability tolerancing, for a start. They are not recommended. In some systems, particularly theatrical PA, 4 pin and other XLRs have been used for bi- and tri-amped or other multiple speaker connections. Such wiring is ad hoc; consult the maker or installer. Use is inadvisable in high power systems, above about 50w to 100w (re. 8Ω), as the current rating of the ‘higher-pinned’ XLRs is greatly reduced.

iv) Speakon The Speakon was expressly designed in recent years by Neutrik (Switzerland/UK) to satisfy the modern requirements of professional and even domestic power amplifier users (where others had failed), and to meet tighter safety legislation (the IEC 65 ‘finger test’, and LVD in EU countries). Figure 9.15 Neutrik’s dedicated Speakon speaker sockets, 8 and 4 pin types. © Canford Audio.

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Choice, application installation and set-up There are two types, 4 and 8 pin, called NL4 and NL8 respectively. Presently, both sizes are plastic housed. While a tough plastic has been used, the experience of several touring PA operators is that they are more easily broken than older, metalbodied connectors (e.g. EP, XLR-LNE series). Due to inadequate shoulder flaring, they are also considered unwieldy by touring professionals, being particularly prone to snag when long cables are tugged while de-rigging. Another annoyance is the locking ring, later changed to a latch, which has fooled many a sound engineer. None of these will bother a domestic or studio user. But then there are the fiddly screw connections, which will demand regular inspection and maintenance as there is no option for soldering. Lastly, when wiring Speakons, the pin numbering can be almost impossible to read. These foibles preclude this well-intentioned connector from universal adoption and professional endorsement in the UK. On the plus side, Speakons are realistically designed to accept conductors up to 6mm2 and cables up to 15mm2, have high torquing hex (Allen) socket terminal screws, are latching, and have solid cable clamping. Contacts are designed to withstand attempted damage by high inrush/outrush currents and arcing. Conventional wiring is as follows (subject to amendment).

Speakon connection (provisional IEC Standard) System category:

Speakon type:

1−

1+

Pin numbers: 2− 2+ 3−

Full range

NL4

F−

F+

(F−

F+): these may be paralleled

NL4 NL8 NL8 NL8

L− L− L− L−

L+ L+ L+ L+

H− ♦ ♦ −L

H+ ♦ ♦ M+

Bi-wired or bi-amped Triamped Quadamped

♦ ♦ M− −H

3+

♦ ♦ M+ M+

4−

♦ H− H− H−

4+

♦ H+ H+ H+

Note: L = LF. LM = Low Mid. M = MF. HM = High Mid. H= HF. ♦ = no connection.

Attention: Copper cables, particularly those with a small number of relatively thick strands ‘creep’, causing terminal screws to slacken in time. To arrest degradation in sonic quality, and avoid risk of fire in high power systems, all connectors (such as the Speakon) based on gas-tight conductor clamping must be regularly checked, shortly after installation, then annually at least. Figure 9.16 Speakon plugs. Note the newer latch on the 4 pin type (left); and the older twist-collar on the 8 pin version (right). © Canford Audio.

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9.5 Output connections v) EP series (originally made by Cannon) The EP series have been widely used for multi-driver connections in touring PA since the early ’80s. They are the 1950s predecessor of the XLR, larger and stouter, and current ratings per pin are proportionately considerably higher. A de facto pin configuration, as used to connect Turbosound and other professional, 3 and 4 band actively-driven enclosures, uses the EP3 as follows: Pin: 1 2 3 4 5 6

Connection: LF − LF + Mid − Mid + HF − HF +

EP4 and EP8 are also used. For wiring, consult the box maker.

vi) The PDN speaker connector This type was a competing precursor of the Speakon, that addressed the safety concerns that were raised with traditional terminals and XLRs. While it appears in catalogues, and offers an adequate electrical connection, it is not known to be fitted to any commercially manufactured amplifiers and is not recommended by many professional operators in the UK.

9.5.1 Speaker cabling A loudspeaker cable’s foremost requirement is to act as if it has little (ideally nil) length; it has to connect the amplifier’s low source impedance as tightly as possible to the drive unit’s terminals (assuming voltage control). To do this, it should have very low resistance and very low series inductance. This is tantamount to saying that the cable should have maximum mutual inductance (magnetic linkage between send and return), and maximum capacitance, these being the dual (reciprocal) elements. A loudspeaker cable that has low enough resistance, but where series inductance is moderate, as is the case with ordinary AC power cable commonly used for speaker connections, will (unless very short, under 12"/0.3m say) degrade the ability of a high performance amplifier to damp the speaker on musical transients associated with all audio frequencies. Figure 9.17 (overleaf) shows how cables’ damping may be tested, using burst signals and a double differential analyser [10]. Loudspeaker cable that has low inductance but too much resistance will suffer a power loss. Long before this has any effect on loudness, it will cause degraded damping at most frequencies. The same series resistance will cause a voltage drop across the cable that follows the speaker’s impedance vs. frequency plot, thereby altering tonal balance as well.

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Figure 9.17 Test set-up to abstract error signals from a typical length of loudspeaker cable, under realistic conditions. Copyright Ben Duncan. All rights reserved

The following table specifies minimum or ideal cable c.s.a., for a notional damping factor of 100, based on 10mΩ per ohm of nominal speaker impedance. In other words, all the cable lengths for the sizes listed will be under 80mΩ for 8 ohms, and under 160mΩ for 16 (or 15 ohm) speakers. For 4 ohm speakers, simply step up the gauge by one or two column(s). The cable gauges required show why it is advisable to feed tandem drive units (as used in many bass enclosures) individually. Never run speaker cables alongside input cables. This might cause positive feedback at RF, hence RF oscillation, which is readily destructive if the amplifier employs BJTs in its output stage. In an ideal system layout, all input cables approach the amplifier at right angles (90°) to speaker leads. Don’t forget that wires running up and down are at right angles to wires running in either horizontal plane.

Speaker Cable Gauge Nominal speaker impedance 8Ω 16Ω

Min Cable size (mm2) for distance: 8' 16' 25' 32' 2.5m 5m 7.5m 10m

65' 20m

(feet) (metres)

1.25 0.75

10 6

mm2 mm2

2.5 1.25

4.0 2.5

4.0 2.5

Inductance Low resistance conductors have greater cross-sectional area (csa) than thin ones. They also have slightly lower inductance, per unit length, but it is essential to recognise that the inductance that counts is the series inductance when the cable conductors are looped. This ‘loop inductance’ is much smaller than the inductance of an individual conductor, provided the send and return conductors are close to each other. Other than reducing the loop inductance (LL), the proximity reduces radiation and confers EMI shielding. Makers of very low inductance cables include Goertz and Kimber (US) and Jenving (Sweden). These types can improve sonics by helping the amplifier’s NFB get a faster grip, and by receiving less RF hash.

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9.5 Output connections Spaced cables Loudspeaker cables with spaced conductors are purely cosmetic. The low capacitance they exhibit is not ideal, as the wide spacing needed to gain it also increases loop inductance, causing smearing in high resolution speakers, particularly those where the drive-units are passively crossed-over and/or don’t have conjugate networks. Such spaced cables are also far more open to pickup EMI, and in multi-driver/multichannel systems, crosstalk.

R-to-L balance (DC resistance / loop inductance) For each cable construction, there is an optimum c.s.a., where increased inductance is balanced against reduced resistance. The higher the speaker impedance (e.g. 15Ω rather than 6Ω say) the less conflict there is between the two, in some disproportion. This is the benefit in connecting paired drive-units via individual cables, rather than one cable equal to the sum of the individual csa’s.

Other effects Skin effect (see also Chapter 5) is a frequency-dependent conductor strangling effect, acting like inductance, but less severe. Its measured and sonic effects come into play significantly, and at increasingly lower frequencies, as loop inductance is reduced. A cable with low skin-effect is not useful unless loop inductance is first low. Proximity effect is another cause of constriction in the cable, so effective c.s.a. shrinks at high currents. It causes heavy gauge cables to under perform at the time they are most required. It can even cause cable resistance modulation, as the temperature rises in the small area of conductor into which the current is confined. If currenthungry speakers are in use, cable designs which combat proximity effect are worthwhile if they also have low loop inductance. Dielectric quality in speaker cables is in many ways far less important than it is in line-level cables, since the cable’s shunt capacitance is highly damped by the amplifier’s output impedance, if not the drive-unit. This is one of the benefits of a high damping factor. Properly researched audio grade speaker cables may also offer purer or specially alloyed or plated conductors; the damping of the mechanical effect of proximity effect and/or air-borne vibration; and lower losses up to low RF, then higher losses.

Shortcomings Purchasers should be wary that gold and copper form bad-sounding oxides, and that super-pure conductor metals oxidise very readily on their skins as soon as the cable is manufactured. For a start, the common types of plastic insulation covering speaker cables releases reactive plasticisers as soon as it is made, that attack the conductor’s surface. The better sounding cables employ conductors that are plated with tin, or made of solid silver or other metals or materials even. They also are clad in a relatively inert insulation, e.g. special PVC with low chloride emission, PTFE or silicone rubber.

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Choice, application installation and set-up Makers of ‘monocrystalline’ copper cables do not mention that the crystalline structure is brittle, and that except with very careful installation, the properties will be lost.

Split options The quality of loudspeaker cable is most significant in passively crossed-over speaker systems. Bi-wiring is mentioned in chapter 2. There may be different optimum types and sizes of cable for the individual frequency band. Connecting each drive-unit via its own cable also reduces each cable’s ‘workload’. In his studio control room designs, the monitoring systems quality researcher (and historic Virgin record producer) Philip Newell takes this one step further. Noting that in speakers with passive crossovers, the power amplifier to passive crossover connection commonly exhibited more sensitivity to speaker cable quality, he moves the crossover to the power amplifier, so the cable length in this section is reduced to a few inches [11]. Again, specific cables might be used for the different frequency bands.

Other hints In large systems, and where XLRs are used for input and outputs, be sure to employ a standard form of speaker cable that is readily distinguishable (by colour difference, markings, size, tactility) from input (line level) cables, in dim lighting, and by people of limited perception.

9.5.2 Impedance setting A minority of transistor amplifiers (e.g. APT, BSS Audio), but most valve amplifiers, have a switch that optimises the amplifier to the nominal speaker impedance. The switch is thus marked usually 16/8/4 ohms, or somesuch. In most cases, taps on either an output- (if the output stage is valve) or power supply- (if transistor output stage) -transformer, are being changed. Mis-setting this switch may temporarily degrade headroom or other sonic qualities, but it should not cause damage. Usually, it is inadvisable to change such switches when programme is playing, certainly at high levels. If a speaker is ‘hard to drive’ (has bad impedance dips), it is possible for sonic quality to be improved with a setting lower than the nominal speaker impedance, eg. 2 ohms for a difficult ‘8’ ohm speaker.

9.5.3 Output polarity Pay particular attention to amplifiers set to operate in bridge-mode. The Channel 1 red or ‘+’ output terminal is usually ‘+’ in Bridge-mode, but be prepared to check the maker’s manual. In absolute terms, the lower channel No’s + is taken to be the ‘real’ +, i.e. polarity refers back without inversion to the hot (or pin 2) input, while the higher channel’s output is usually the one that is inverted.

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9.6 Placement and fitment

9.6 Placement and fitment Care ! Many high power or highly specified audiophile power amplifiers are heavy enough to damage or stress connective tissue in your spine or other places, if they are not lifted with due care. Always bend your knees and lift by unbending here first, transferring the weight gradually to the spine. Reverse the process when letting down, ending up with bent knees. Unless you are a weightlifter or PA rigger, quite a few highly rated power amps are too heavy to lift comfortably on your own − at least two lifters will be needed. In warehouses, on stages and in demo rooms, handling aids, such as trolleys, wheelboards and carts are used. These are urged for moving weighty power amplifiers in the home also. When installing amplifiers into racks (usually for touring), the racks are normally faced up, and are often on a wheelboard, which is locked. Rigging gloves (with part open fingers) are worn to protect fingers when letting down a heavy beast into the rack. With heavy amplifiers, and whenever weight distribution is rearwards, rear rack fixings are essential. These should be supplied by the maker. In domestic and studio installations, the sonic performance of power amplifiers may be improved by avoidance, damping and/or ‘grounding’ of air- and structureborne vibrational energy. For perfectionists, this means (i) evaluate sonics in different locations, with different types of music, at different sound levels. Moving just 12"/0.3m away from a corner or from a wall, can be significant. (ii) then evaluate damping and conductive aids, e.g. ‘Blutak’, foams, dampers, mats, spikes, etc. (iii) then re-evaluate the new, best position.

9.6.1 Cooling airflow conventions Most domestic amplifiers are cooled solely by convection. Fan cooling is only permissible if the fan noise is extremely low at the listening position. The further ear-to-amp spacing possible in a large room helps. In the highest quality designs, a considerable volume of cooling air is moved with much lower SPLs than is the norm, using a centrifugal fan. The more refined professional amplifiers have also long had continuously variable ‘servo controlled’ fans, which with a class B, A-B, G or H amplifier, only speed up when there is substantial current, generally when the programme level is quite high. The snag is that high speeds may persist for some time after a loud passage is followed by a quiet one, or silence, and the ears gradually notice the fan noise. At least any changes can be gradual. Some designs have a slow setting and enough heatsinking capacity to handle occasional loud passages, while keeping fan nose uniformly low. A third kind have two fan speeds, viz. slow, switching up to fast if heat exchanger temperature gets into emergency realms. This type causes abrupt changes in fan noise, making them highly intrusive in the listening room.

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Choice, application installation and set-up Any amplifier’s cooling can be defeated by simply racking it with other amplifier, because the direction of forced air isn’t standardised. In one standard, call it ‘A’, air drawn from the rear emerges from the front. This helps users know that the fans are working, and by assessing the waste air temperature, one can gauge some idea of the amplifier’s work condition. Also, the back of the rack may be kept cool. Other makers employ standard ‘B’: they draw air in from the front, and exhaust from the rear. If the rear of the rack is inadequately ventilated, so the main ambient air temperature is lower, this yields cooler running. On the minus side, it stresses or can prove fatal to other amplifiers in the same rack employing the reverse airflow direction, and also any equipment that lacks forced cooling, especially if it runs hot and depends on convection cooling. A third type of amplifier, ‘C’ has no front venting. Hot air is exhausted or drawn in from the sides and lid, and vice-versa from the rear panel. When racked, the rack sides should ideally be vented (as a chimney or intake) at top and bottom. If not, the rear of the rack should have a substantial volume of cool air or draught. Figure 9.18 The modular heart of BGW’s PS2 budget professional amplifier. The BJTs (not visible) are cooled with enhanced reliability by the lateral, extruded-aliminium heat-exchanger. A small fan blows air at the centre of the fins. Most of the air exits at the sides a type ‘C’ airflow. © BGW

Figure 9.19 The Rauch DVT 250s , another example of type C airflow, but with a rear intake. Note bi-polarity true error indicator LEDs above the knobs, covered in chapter 5. © MS&L.

The temperature of the waste air from the small number of very high quality amplifiers employing centrifugal (alias tangential) fans, which move air slowly but at high pressure, can be hot enough to burn users. When hot air exits from the rear of a rack containing such amplifiers, take care that there are no flammable materials (newspapers, old polishing rags) present. Any PVC cables passing up the rack must be dressed to one side of the hot air path.

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9.6.2 Cooling and air pollution Fan cooling concentrates air-borne dirt inside amplifiers. A filter should be used but if the air is dirty, this soon chokes the air flow. This in turn stresses the common axial type of fan, reducing its lifespan, while thermal headroom suffers. Cleaning a filter is easy enough; gently wring out in hot water laced with a spot of detergent. When many amplifiers are in use, it is expedient to filter the incoming air, if this can be hermetically connected. The large filter will be easier to maintain and the individual amplifiers’ filters will need checking and cleaning less often. Fan filters are sometimes removed, often by those who have never had to clean out an amplifier. The parts that then get dirty eventually have to be cleaned, and are far harder to clean, and prone to be damaged. For example, air also contains some moisture, which is liable to be acidic or corrosive both in heavy industrial and urban areas, and within 10 miles of the sea. Also, since 1980, smoke machines have become increasingly de rigueur in many musical and performing art circles. Some of the oils have proved quickly and fatally corrosive to amplifier electronics. Fan cooling exacerbates the deposition of corrosive substances, and as most PA amplifiers have to be fan cooled, any working near to smoke machines for prolonged periods can end up costing the user or owner a new set of PCBs and a major decontamination exercise. Air intake filters provide little protection against smoke and corrosive chemicals. Spraying an elastic protective (‘conformal’) coating over all the parts can provide some protection, but unless the coating is known to have sealed every crevice in every corrodable part inside, amplifiers have to be kept away from the worst of the smoke. Otherwise, the highest quality fan-cooled amplifiers, particularly those made for the stage and installation in other arduous environments (e.g. by the sea, or in desert areas) have air passages that solely pass through anodized aluminium, stainless steel, or some other chemically inert surface. Convection cooled amplifiers used in most domestic systems and some professional setups can have internal or external heatsinks. If external, they may be casually fan cooled if desired (for a loud session) without worries about filtering the air, by simply directing a suitable fan at the fins. If the heatsinking is internal with louvres, or particularly top venting, there is again a dust and chemical pollution problem unless the cooling chimney is substantially separate and sealed from the remainder of the circuitry. The solution is to clean the air in the room, and avoid opening windows when outside air quality is low. This is good for the listeners’ health too. With convection cooled amplifiers, the user can choose how well they are cooled. Cooler running will increase reliability. To achieve this, arrange the siting or mounting so air can freely circulate around the case on most of all sides. If stacking, put substantial spacer blocks or bricks between amplifiers.

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9.7 The 1 to 5 of prudent amplifier use 1. Turn on audio power amplifiers last − and the remainder of the system first. In high resolution systems, preferably only connect amplifiers inputs after the source system has settled, e.g. after a few seconds for well designed transistor equipment. 2. After 1, and only then, connect the speakers after the amplifiers and everything preceding them, has been switched on, and all have settled. This is a wise precaution when valuable loudspeakers are connected to an amplifier which lacks DC protection and turn-on muting. It will prevent the transmission of loud thumps or bhlatts. Don’t use the power up/down switch on an amplifier or preceding equipment as a mute button, unless previously proven, as poorly designed units have been known to produce wild or embarrassing sounds up to several minutes after being switched off. 3. Keep as cool running as possible. If fan cooled, check and clean filters regularly, according to need. 4. Clipping damages or harms human hearing and loudspeaker drive units alike. Reduce levels immediately. 5. Before turning off the system, disconnect the speakers or set the amplifier to mute. Then switch off the source equipment. After 2 minutes settling time (see Section 3.5), the amplifiers may be switched off. [Note: Steps 2 and 5 are not practical nor often necessary in large and professional systems and will be waived in such cases].

9.7.1 System back-end troubleshooting Buzzing or humming Check input connections. Try other cables. Does the buzzing stop when the source equipment is disconnected ? What does this tell you ? Try to use balanced connections whenever possible, even if your source is not balanced.

Coloured sound (In active systems): Check cables, amplifiers and drive units in missing or amiss frequency range(s).

DC fault When an amplifier’s power or driver transistors fail short-circuit, to cause a large DC fault, the first symptom is a raspy buzz, as the DC supply’s ripple is fed directly to the speaker. At the same time, the cone will move largely in or out. If this condition persists for long enough for you to realise it, i.e. above one second, then either the amplifier omits a DC protection relay, power shutdown, or other means of stopping the DC power source, or the protection isn’t working very well. Either way, the speaker cables need disconnecting immediately. Then switch off the amp and leave it to discharge. If the amplifier is 100% direct-coupled, do not be too hasty to blame it, as the DC fault might just be in the preceding gear.

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9.7 Prudent amplifier use Fuzz When heard on decaying notes, and during quiet passages, may be (i) crossover distortion or (ii) a rubbing voice coil. A second speaker proves which.

Hiss Not much should be audible when the amplifier’s input is disconnected and shorted. If prominent, it may signify a sick semiconductor or RF oscillation.

Intermittent sound Look for a mechanically loose connection. First, try exchanging cables. If this fails, disconnect the power, allow time to discharge, then try turning the amplifier on its sides, and over, listening for loose items. If this fails, look inside.

Noise If really a hum or a buzz, rather than hiss, see “Buzzing or humming”.

Overheating If an amplifier overheats (and shuts down), or the casing becomes hotter than usual, then check: (i) the speaker impedance – is this within ratings? (ii) the connectors and cabling – is there a partial, possibly intermittent short? (iii) the heatsink cleanliness. If inside the casing, with a chimney vent, or fan, when was the dust last blown out? Is fan airflow enough? (iv) signal – is this clipped or unusually compressed? (v) stability, especially when the amplifier isn’t being driven hard. Use a ‘scope or RF sniffer to check. If the amplifier overheats with no signal, then RF is a most likely culprit. Overbiasing of a class B or A-B output is also possible.

Polarity If polarity testing reveal inconsistencies, first check (by eye, and with the help of a continuity ‘bleep tester’): (i) the output cable and connector wiring. Does the brown (or whatever colour) wire follow through from the amplifier’s + terminal to the speaker’s + terminal without transposition? (ii) the amplifiers’ input wiring. Polarity transposition is most likely when different amplifier models are in use, particularly if they have balanced inputs and/or different XLR connection standards, i.e. is pin 2 or 3 hot? (iii) perhaps the amplifier is inverting. This is reare, but quite possible. If no inconsistencies are found, check elsewhere in the system wiring. Are the speakers consistently wired ? A polarity checker is often best used, as at least one loudspeaker maker, JBL, has an inconsistent definition of the ‘+’ terminal. Don’t ignore the possibility of nested or other multiple, interactive faults. Before settling on rewiring, be aware that the polarity (or ‘phase reversal’) switching on an active crossover can solve (or cause!) polarity errors at the push of one button.

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Choice, application installation and set-up Weak sound Check LEDs indicating shutdown or malfunction/abuse have not illuminated. Check the gain control settings, if any. Check the power up/down muting system has un-muted. Try re-powering. If the signal present LED is dim or out, check the input connections and cabling. If lit, check output connections and cabling.

Weak, distorted and intermittent sound Check for whiskers of wire shorting inside plugs or between damaged conductors. When tracing faults, be thankful for the redundancy of the typical audio system. Most sound systems have two or more amplifier channels, and two of almost everything else, invaluable for comparison. Work by tracing the signal, or absence of signal, from one, the other, or both ends of the faulty part(s) of the sound system. In a large system, time will be saved by checking in the middle too. Don’t forget that faults can be ‘nested’, i.e. two or more independent events have been known to occur. When this happens, a sure cause (like a bad connection) is found and rectified, but the problem persists.

References 1 2

Atkinson, John, When things go wrong it hurts me too, Stereophile, Feb 1989. Pirsig, Robert., Zen and the Art of Motorcycle Maintenance, Hodder & Stoughton, 1976. 3 Duncan, Ben, Ultra high power amplifiers: the rationale, Proc. I.O.A., Reproduced Sound conference, Nov 1993. 4 Bacon, Mark, Contact basics, Speaker Builder, June 1990. 5 Duncan, Ben, The mains issue, Hi-Fi News, Dec 1991. 6 Duncan, Ben, Black Box, about mains ecology, 7 parts, Hi-Fi News, Nov & Dec 1995 & Jan to May 1996. 7 Duncan, Ben, PSU regulation boosts audio performance, Electronics World, Oct 1992. 8 Giddings, Philip, The proper use of grounding and shielding, S&VC, Sept 1995. 9 Lampen, Stephen.H, Shielding the information, S&VC, Sept 1995. 10 Duncan, Ben, Loudspeaker cable differences, case proven, Proc. IOA, Vol.17, part 7, Nov 1995. Also in Studio Sound, Dec 1995 and Stereophile, Dec 1995. 11 Newell, Philip, Monitoring systems, Pt.9, Studio Sound, Jan 1991.

Further reading 12 Don & Carolyn Davis, Sound System Engineering, Sams, 1975. Appendix III, Recommended wiring practices.

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10.1 Classifying failures When an amplifier needs repair, the cause is usually one of the following:

Electrical abuse damage This is the most common cause when the less rugged kind of solid-state, mainly audiophile-grade domestic power amplifier, which stands little abuse, needs repair. The most common victims are output and driver power transistors and associated, fry-able parts, usually resistors. Such equipment commonly omits high temperature warning, thermal protection, effective DC protection, discriminative fusing, power up/down muting and other stress-relieving or protective mechanisms. When high sound quality is being provided at low cost, this understandable. Alas, the maker often neglects to remind the user of this. For 95% of careful owners, this has no consequence. However, if an amplifier is driven into difficult speakers, and backed up to a radiator; or driven relentlessly for several hours for a party, and then overheats, there is no safety net, and it duly expires. This saves n-thousand careful or lucky users (say) £9.37 on purchase, but costs say 50 unlucky users each £120+, in repairs and shipping. In some cases of electrical damage, it is endlessly debatable whether the maker or the user is at fault – irrespective of the local ‘consumer’ law.

Physical wearout Excepting valves, little wearout will usually occur until an amplifier has been in use for over 5 to 10 years. The parts most likely to actually wear-out in a physically obvious way are pots, panel legends, switches and poor quality or roughly-used connector contacts. Less apparent, the most wearable parts inside include main reservoir electrolytics, (surviving) output semiconductors; and zener diodes and resistors that routinely run quite warm or hot to touch.

Maintenance and surgery To parts that become hot, cycling, where equipment is turned on and off every day, causes wearout faster than continuous operation. For this reason, prudent owners of installed (cf. touring) systems leave their amplifiers switched on at all times. Sonic quality may improve as a result, but standby energy consumption also becomes an issue. As an amplifier grows older, certain birthdays count. If you do not know the date of the amplifier’s birthday, look for date codes on electrolytic capacitors, e.g. 3194 is a capacitor made in week 31 in 1994. So the unit won’t be older than this (unless the capacitor is a later repair), but it might be a year or two younger, with parts lying in stock. Weekly or monthly Check and clean fan filters (mainly touring). 1st birthday and annually If casework open, blow out dirt. If fitted, check and clean air filters. Check all connectors and switches. Check electrical and mechanical screws and nuts all ‘firm tight’. 7th and 14th and 21st birthdays Replace main reservoir electrolytic capacitors. Check other elcaps and fans. Replace fuses. All output stage and driver transistors may be replaced if they are BJTs, and have been cycled and/or hard driven. Expect some random failures.

Mechanical damage All but a few power amplifiers are heavy for their size. At least they seem it in comparison to most other audio equipment, excepting some speakers and reel-toreel tape machines. Even with all the weight-saving advantages of transistors, some professional and high-end power amps are amongst the heaviest electronic equipment packed into 19" rack-sized boxes, requiring two strong people to lift move them very far without handling aids. By contrast, most electrical and electronic panel components are delicate. Even supposedly ‘heavy-duty’ controls and connectors can be snapped, distorted or otherwise trashed when a heavy metal box is dropped onto a solid surface, or just awkwardly handled. With a lightweight unit, if somebody accidentally knocks or kicks a switch or other panel object the unit will probably move back and the force will be dissipated, so quite delicate switches give no problems; but on a heavyweight amplifier, the box isn’t going to move, so the component takes all the strain. For domestic amplifiers, mounted on a deep and stable shelf, or otherwise secure in situ, and excepting little fingers, the worst physical abuse is likely to be arising from inadequate packing and transport by a careless carrier, in the event of moving house; or having to send the equipment away for fixing. This shouldn’t happen if the original packing is saved and used.

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10.2 Problem solving procedures If the fault is mechanical, what needs remedying will usually be obvious enough. Otherwise the problem is almost certainly damaged electronics, and invisible as such. As with testing any audio electronics, power amplifier troubleshooting may begin with some combination of: (1) Trying out and listening with different sources, speakers, cables, etc. Section 9.7.1 gives some examples of symptoms. (2) Visual inspection. (3) Critical component testing. (4) Inter-channel or inter-unit measurement comparisons. In depth testing is ideal (see chapter 7) but is often not essential, if skilled listening can detect no problem. (5) Provocation and monitoring. See chapter 8. Procedures 2 and 3 are covered in the remainder of this chapter. They will deal with the majority of power amplifier faults, which involve outright component failures, rather than subtle defects.

10.3 Universal repair procedures In the following sections, the repair of principally solid-state audio power amplifiers is covered. It is assumed that there are no schematics or other service manuals. If so, they take priority, but the following will still prove informative. A suitable, creative merging of the information by the reader is expected, since in reality the active components may have diametric roles to the ones described (e.g. a MOSFET front-end to a conventional BJT output stage), giving more permutations than there is the space to deal with individually. New, specialist items mentioned in the following procedures are described in the subsequent sections 10.4.1 - 10.5.3. If an amplifier is said to be ‘dead’, begin by disconnecting the mains power and removing the cover(s). Allow 5 minutes to discharge. Check DC rails (across all large capacitors) and discharge with bleeder if above +½v. It’s assumed for now, for simplicity, that all active devices are BJTs. If not, see also section 10.5.2.

Observations In a naturally well lit room or using adjustable bench lighting or a small hand torch, make a careful physical inspection, looking for any of the following. This may require several separate passes: i) Blown fuses. If glass bodied, eye inspection is often enough. If ceramic, and to be sure in all cases, check to see if there are any DC volts across them. If nil volts, only then check with a continuity or ohms tester. If a voltage is present, discharge it with a bleeder resistor – see end of section 10.4.2.

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Maintenance and surgery ii) Damaged or completely missing PCB copper tracks. Look for the telltale ‘shadow’, and the torn or melted copper edge at one or more ‘ends’. iii) Discoloured, browned or charred parts. Also a nasty mess. This could be signs of chemical emission from electrolytic capacitors and wound components; a decaying mouse; various kinds of ‘biological waste’; or simply the remains of a spilled, sugary cocktail drink that has collected several years’ dust. iv) Any flux residue across tracks that is darker than the majority. If you can see any, gently scrape or pick it off. v) Loose screws or nuts. Also shake the casing, looking out for free bits of wire, or any other loose parts, usually screws, offcuts of wire, hairpins and plectrums. vi) Loose feeling connectors, switches, pots, etc. Wiggle all large PCB mounted parts including elcaps. If the soldered pins or leads seem loose, the joint is probably dry. Remove solder and remake. vii) When archaic, sub-standard, fragile ‘Paxolin’ PCBs are used (as employed in nearly all mass-market products made in the Far East), look for cracks, particularly radiating from hard fixing points. Test potentially affected PCB tracks for continuity, repeating while flexing or tapping the PCB.

Decisions If an obvious, minor cause has been found and sorted (such as the later items v to vii) and nothing catastrophic appears to have happened to the amplifier, pass along to ‘First repowering’ (page 387). Otherwise, assuming signs i - iv are discovered, surgery will be needed.

Preparations The first task is to remove the PCB(s). At this point, it’s important to make notes and sketch plans of any wiring that has to be unsoldered or looms that have to be unplugged. Sometimes self-adhesive labels or a variety of different coloured spiritbased felt-tips can be used to code connections. Beware of washing such marks away later with a cleaning agent. Also take care to retain nuts and bolts removed during dismantling; placing them in a labelled, resealable plastic bag. Once the sick PCB(s) have been removed, the chassis can be put to one side for a while. A de-soldering ‘gun’ will be needed, to help remove soldered parts with a reduced risk of damage to the PCB. Desoldering braid is an alternative that can cause less damage in some circumstances. When desoldering proves troublesome, a sharp rap can help disgorge deeply entrenched solder. If the copper track is lifted by the desolder gun’s suction, try reducing the iron temperature, or use a smaller gun.

Cleaning-up ‘Burnout’ in an amplifier made of bipolar transistors is often literal, involving a component fire. Such fires are usually self-extinguishing but even when very localised, they can make, like building fires, a disproportionate mess. If there is

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10.3 Universal repair procedures charring and soot, the first job is to remove this. If done gently by brushing while the PCB is nearly vertical with an aqueous aerosol foam, or else an alcohol-ammonia mixture (i.e. Hurd’s No.1 Grimewash – see section 9.35), this reduces the risk of loosing clues as to the value or part number of the charred parts – most helpful if no service data is available.

Burns surgery Where the PCB is fibreglass and has caught fire, the hard surface is lost and the core turns into a mass of spongy, carbonised fibres. These are conductive, shorting all the connections they contact. For safety, a burnt PCB should be scrapped. Such a board should be used only for parts, alias ‘cannibalisation’. If the burnt area is small, the price of a new PCB (with parts mounted) is uneconomic or a new PCB unobtainable, or in an emergency, the board may be safely reused, provided the carbonised area can be cordoned off, by scraping away all the loose carbon, and cutting away tracks that enter the zone. A mini-fretsaw might be used to cut around the carbonised fibres. These tracks must be re-routed with suitable insulated wires, and any components will need reconnecting in mid-air, and securing with silicone compound, or more securely by mounting via fresh holes, drilled on an unburnt area.

Lost values If the amplifier has more than one channel, then obliterated parts may be ascertainable from the other channels, with due care being taken if the layout is in any way different, even just symmetric. Failing this, don’t guess. Contact the maker, importer or their agent for advice and information on service information and spares; or place an advert in a sympathetic magazine, newsletter or bulletin board, looking for either service information, or else sight of a working example from which values can be ascertained.

Rs and Cs If for some reason spare resistors and capacitors are not available from the maker or agent, then not being able to obtain exactly the parts used is not an insuperable problem. If the unit is of audiophile quality, consult specialist audiograde part suppliers as to equivalents or upgrades on the parts used. Generally, you can replace a given wattage or voltage rating with parts having higher wattage or voltage ratings, so long as the % tolerance and quality of the part is also similar. In most cases electrolytic capacitor values (in µF) can be considerably increased without concern about the limit, again provided quality is approximately comparable. Other, mainly non-electrolytic-type capacitor values and some resistor values in conventional power amplifiers are relatively critical, some (not all) within +/–10% or less. These should not be varied unless there is good reason. Some resistor values are more likely to be critical in amplifiers with low or nil NFB.

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Maintenance and surgery To be safe, resistor wattages and capacitor voltage ratings should never be reduced, unless it is known that the maker had over-rated the original part for example, to overcome a sourcing difficulty. It is unlikely that a maker will fit anything overrated except under the shadow of a prior bad experience.

Output Network The OPN (OutPut Network) is a perennial weak spot in semiconductor amplifiers. The resistors here can smoke, burst into flames or even explode if driven with sufficient HF/VHF level. It’s easily done by accident when playing a test CD, or sweeping a test generator. Or RF may be present on the incoming signal. In amplifier servicing, discovering slightly singed zobel resistors is not unusual. They are best looked at, whenever amplifiers are overhauled; and also after anyone discovers that a faulty console (or preamp or other equipment) has been driving RF up the system. Thoughtful makers will place the Zobel resistors where they can be readily inspected and replaced. Ironically, once the Zobel network is at all damaged, the stabilising resistance value usually increases, thus the amplifier is all the more likely to go unstable, generate its own RF output, and finish off the burnout.

Transistor testing After re-instatement of damaged passive components, the next step is to test the transistors. As the destruction generally begins in the output stage, it is most productive to test by working backwards from there. Testing bipolar transistors is most reliably carried out with them isolated from the circuit, i.e. removed. This does not mean it is essential, just preferred. If removal is impossible, some testing can still be accomplished. In either case, basic Go/No-Go testing is often all that is needed. See section 10.5.1. With TO3-cased power transistors, be aware that even with the base and emitter wiring removed or unplugged, a low resistance can occasionally occur if the leadouts are unsleeved and therefore capable of touching the thickness of heatsink metal through which they pass; or if solder or wire slivers held in place (and possibly hidden) by thermal grease, are making devious contact.

Other active tests Diodes, zeners and op-amps may need testing also. Op-amps are rarely implicated except as prime causes (very rare with good, modern ICs), or input overdrive or fire victims. If there is no evidence of an IC having been harmed, and especially if the PCB has plated-through holes, then it is best left alone until everything around it that is more readily tested has been proven good. If replacement is necessary, always replace with a high quality IC socket so the problem of desoldering does not recur. On a D/S (double-sided) PTH (plated through holes) PCB, do not attempt to desolder an IC unless you have the proper vacuum tool. Instead, nip the legs flush to the board, very gently. Then there is a slim chance of being able to use the IC again. The amputated legs can then be desoldered and removed individually.

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10.3 Universal repair procedures As two legged parts, zeners and diodes are easily tested in isolation by desoldering and extracting one leg alone. See section 10.5.3.

Spares sourcing Resistors, capacitors, diodes and zeners occur in a limited number of internationally agreed values (e.g. 400mW, 500mW, 1.3w and 5w for most zeners) and have simple specifications which make substitution anywhere in the world straightforward if the need arises. Transistors have far more parameters. There are no standards but due to the use of negative feedback, and other error-correction schemes many are 100% interchangeable. Appendix 3 lists the key data on many widely used audio BJTs. For more information, the standard reference is Towers [1]. Except where designs have been pushed to the limits, or where very close performance matching is essential, replacement transistors of different makes and types having notionally similar specifications often show little if any audible or measurable deviation in performance. Differences may be practically indefinable – except possibly over a large population. This is a useful benefit of amplifiers which use some global NFB. Then satisfactory replacements or equivalents can be obtained from the various specialist audio and electronics parts suppliers.

Special semiconductors For a number of professional and high-end designs, where the maker has selected special parts, and even gives them special numbers, replacement transistors are best first sourced from the makers or agents. Special selection allows high performance to be attained consistently, despite low or zero global NFB. The selection process raises costs though. Imagine 1000 transistors are tested for 6 things. Imagine 10% are rejected in each test. That leaves 53%. The price each has now nearly doubled, for any given % profit. Now imagine one additional test, the yield is a far from unreasonable 5% (of the 53% or 531 remaining). Now the price for the 26 items would need to be thirty eight times the original cost. The upshot is that High-End semiconductor spares are special and can cost plenty. Provided the maker can convince you that he has done the selection testing, and that the out-of-spec devices cannot be sold to amortise the selected part prices, then very high prices are a statement mainly about the wide tolerances of semiconductors and the benefit of NFB. These same high prices, and the ease with which even experts can occasionally vaporise silicon by ‘one inadvertent mistake’ makes it safer for all parties’ feelings, that such repair work is carried out by the maker or retained agent. A few makers have abused the system. The transistor numbers on one well known British PA amplifier were re-marked with the company founder’s car registration number, but the transistors were quite ordinary.

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Maintenance and surgery Allowable equivs The key voltage rating is Vceo. This should be at the very least 10% higher than the sum of the supply rails, e.g. +/-45v rails sum to 90v, so a Vceo of 100v would be (just) safe. A higher voltage is preferable and any higher rating is ok on this basis, but going too high (more than x1.5..x2 the minimum) may compromise other parameters. Vcbo is a common voltage rating condition that may appear if Vceo is not shown. It can often be 20% to 30% higher than Vceo, so derate accordingly, e.g. If Vcbo is 120v, Vceo may be under 90v. The main current rating (in mA or Amps) is IC . If many transistors are paralleled for redundancy, a slightly lesser IC rating may be passable. Otherwise it must be the same or higher. Again, too much higher (say 1.5 to 2x or more) will likely conflict with other parameters. If there is any doubt, the SOA curves of the original and replacement parts should be located and compared. FT (transition frequency) is an indication of open loop frequency response and ‘speed’. It is a relatively loose parameter. If the fT of a replacement part is more than 50% lower or higher, RF stability and/or HF distortion may be affected. This might be latent until a particular combination of load and temperature occurs. So picking a part with a similar fT (within 50%,150%) is advisable. If not, choosing a higher fT will also avoid increased distortion or limited power gain at high audio and ultrasonic frequencies. HFE (or Hfe) is similarly loose, +/–50%. If comparing the values, note the test current, which must be roughly similar (in A or mA) to make the comparison. It is more useful to match any new parts that are being paralleled to each other and to the Hfe of any surviving original parts, rather than meeting the relatively arbitrary textbook specification. If a transistor type is unobtainable, unknown, or far too expensive for no apparently good reason, a knowledge of the circuit can allow a standard type to be designed-in. Otherwise, spares may be had from cannibalised equipment. However, if the equipment is over 10 years old, then with output transistors (and other wearable parts like elcaps), there is a risk of fitting parts that are entering their wear-out phase.

Reinstatement Before replacing power devices, clean down their mounting positions with Grimewash (page 349). Also ensure each mounting area is smooth and level. Any roughness must be gently ground or rubbed away. Old insulating micas or rubbers are best thrown away. If re-use is the sole option, clean and inspect carefully. Rubber insulator pads shouldn’t show signs of stretching or tearing. Micas should be cleaned by shaking in a jar of solvent and inspected for tiny holes and splittage. Unless these occur on the protruding edge, the mica is unsuitable for re-use. Until recently, white zinc-oxide-loaded thermal conductivity compound or ‘grease’ was an essential, messy nuisance in any high performance output stage assembly. It is needed to fill in the microscopic hills and valleys, between the mating surfaces, to greatly improve heat conduction between surfaces that appear flat to the human

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10.3 Universal repair procedures eye. If it isn’t available, silicone grease or even car-grease can be used in emergencies as inferior substitutes. But as Zinc Oxide compounds are inexpensive, substitutes are not recommended. In recent years, rubber-based insulator washers have been developed which don’t employ or require grease. The best types can (at the time of writing) slightly outperform greased mica. These types are highly recommended, since Zinc Oxide grease is unpleasant to handle (though of low toxicity) and a regular cause of further fatality – as it can hide particles that prevent proper seating , and wire slivers, that can cause shorts at a later stage. For these same reasons, if grease has to be used, don’t be tempted to ‘economise’ and re-use what is around by re-spreading it. In any event, spread it thinly exactly where it is needed. A flexible, glue-spreading spatula is best. Only a few thousandths of an inch is needed. Any excess – which then exudes from the edges and spreads all over – must be cleaned away. Power device insulation also depends on nylon bushes. Replace them if they are crushed or damaged in any way that reduces their insulating thickness or integrity. When heatsink-mounted transistors are re-mounted, correct tightening is essential. A torque wrench should ideally be used, set to between 60 and 90cN×m, or 6 to 8inlbs or else tighten by hand so the tightness is ‘slight firm’, i.e. not particularly tight. Be aware that initial tightness relaxes after a few hours. Preferably after a few hours, go over all the screws to check the tightness is about the same. Nail varnish may be used to lock after final test. Ideally, correctly sized Belleville washers should be used on all power devices to preserve the optimum torque band. When replacing the remaining, usually PCB-mounted semiconductors, take care not to stress them. The leadouts of metal can parts are not generally strain-relieved inside. They are therefore advisedly mounted on plastic (and stood-off by) spacers. Be sure to re-use these. Plastic cased transistors are more rugged in this area; but all transistor leadouts are easily fatigued if bent hard (>45°) or more than once or twice: look for tiny cracks if this happens. Such a fatigued part is good only for scrap. Some PCB mounted parts are mounted on or bear small heatsinks. Often, this class of heatsinking is un critical. If in any doubt however, treat as being as critical as the output stage, use heatsink compound. Particularly if the parts are plastic, torque correctly.

Other checks When bipolar output stages require repair, certain associated resistors are commonly implicated. It is possible for these to appear undamaged when they are actually open circuit or have ‘gone high resistance’. Check the following: (i) Low value (0.1 to 1 ohm) OPS sharing resistors, typically 1 to 20 watt rated, and usually wirewound, in line with emitter or collector terminals (Re in Fig 4.14). Usually mounted close to the output devices. (ii) Low value (10 to 220 ohm) ‘flushout’ resistors between OPS base and emitter (Fig. 4.6). May be close to the output devices or on the driver PCB. (iii) The resistor(s) in the output Zobel network(s) (Fig. 5.32). Usually 5 to 20 ohms, 1 to 20 watts, may be carbon composition, metal or carbon film, or wirewound.

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Maintenance and surgery PSU pre-testing In most high performance amplifiers, damaged power supplies are remarkably rare and non-coincident with blown output stages. Still, before proceeding, prudence dictates that the PSU should be checked if it is easily disconnected from the repaired amplifier circuitry. Often this is easy, because the two, ‘hot’ DC supply wires (usually red and black – but not always) are either pluggable (e.g. at the bridge rectifier) or can be un-clipped or un-screwed (e.g. at the elcaps). Be sure to insulate all bare, loose wires! If there is no other means of isolation and test connection, there may be DC rail fuses. These may be removed for isolation. Section 10.4.2 outlines gentle powering up techniques, including use of a Variac. If when testing the power supply, the Variac’d AC supply sags, or the series bulb lights, turn off and check for: (i) correct AC voltage setting. (ii) healthy reservoir capacitors - no signs of chemical leakage. If they have been disconnected, are the wires replaced in 100% the correct places and to the correct polarity terminals (irrespective of physical orientation) ? (iii) healthy rectifier bridge. It is assumed the DC feed out from the bridge has been disconnected. Then if the bridge is internally shorting, the high current will cease when the AC input to the bridge is disconnected. In most cases, this means isolating both feeds. If the problem is not found in the above, but persists, the mains and PSU circuitry should be isolated and systematically checked with a DOM (or other ohm-meter) for shorts or insulation breakdown. When using a Variac, this is a good instance to determine and note the minimum supply voltage at which the amplifier will turn-on. All is well if the power supply (after being isolated) can be powered up and delivers a DC voltage that is in proportion to the AC input. If a lightbulb is used, the AC input voltage on the bulb’s output side will need monitoring. If the supply is say 190v, then relative to 240v, the DC rails would be about 79% of normal, eg. +/–35v, if normally +/–45v. After testing, discharge the supply (see end of 10.4.2) and reconnect it to the output stage and/or reservoir capacitors.

More caveats ‘If it ain’t broke - don’t fix it’ In a stereo or multi-channel power amplifier, one or more channels may be unaffected by damage. To protect these from unnecessary stress and disturbance during testing, you may choose to isolate the DC power to this/these, until after the repair is proven. If powered by wholly separate power transformers, it may be simpler to disconnect their AC input feeds.

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10.3 Universal repair procedures Bias presetting Power amplifier biasing is covered in chapter 4, notably 4.2.5, 4.3.3, and 4.6. Class G and H amplifiers are biased as class B,A-B or A. Under-biasing isn’t harmful but sonic quality will be degraded, most at low SPLs, and at HF. Over-biasing can be fatal to amplifiers with BJT output stages. However, with cautious setting, there will be no problem. Turn preset slowly and pay attention. Before proceeding, the biasing preset should be identified. This will be present in most Class B and A-B -based designs, and absent in most Class A designs. If there is no information, the preset is the one nearly always coupled to a small signal, NPN bipolar transistor, connected as a Vbe multiplier or Vbex, which is usually connected to the main heatsink and electrically between the driver transistors – or other stage, immediately after the VAS. See section 4.3.3. If there are a number of presets do not guess. Either get the maker’s information; or work out which does what by tracing-out the surrounding circuitry. Optimum biasing of bipolar transistors requires a harmonic distortion (%THD+N) analyser. The bias for a new set of transistors will almost certainly require resetting. Failing this or any other distortion test equipment, it is possible to set biasing by iteration with just a milli-ampere meter. First, turn the preset so bias is at minimum. In most cases, this means turn the preset ‘down’, i.e. CCW. This is not always true. If unsure, and there are no diagrams or PCB legend giving this information, set halfway, soft power the amplifier (via a lightbulb) and immediately establish at switch-on whether current is reduced by turning CW or CCW, by watching the bulb’s intensity. Biasing of amplifiers with MOSFET output stages is usually class A-B or A rather than B. In practice, this means at least 25mA and up to 100mA for each pair of MOSFETs. As biasing is relatively non-critical, there may be no preset. There is then nothing to worry about so long as the heatsink on the repaired channel runs (when quiescent) at a similar temperature to another, intact channel. 15ºc above ambient is usual.

First re-powering In the following initial tests, it is assumed that no signal is being applied and there is no load. If a bulb alone is used (page 391), current can be crudely monitored by noting its brightness. Apply the power. If after a brief build-up to brightness, the bulb dies back to a dull glow, if any at all, all is probably well. If a Variac is use, bring up the voltage slowly to the minimum established (above). If the series bulb again initially lights, but dies back down, all is well. If the bulb stays lit, turning the bias preset down in the previously determined direction, should reduce the brightness. If it does not alter it, there is a fault. Turn off immediately and recheck the repairs and any re-made connections. Likely causes are a reversed connection, or a missing or wrongly placed insulator.

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Maintenance and surgery Safe bias tactic Once (with AC power applied) bulb brightness can be controlled by varying the bias preset, accurate metering is worthwhile carrying out. Switch off and allow the DC rails to discharge. Disconnect the +ve wire to one amplifier channel and insert a mA meter, or else a low resistance ( ⇒ ≤ ≥ –∞

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One hundred. Two hundred. One thousand. One million. One thousandth. One millionth. Millionth of a millionth. Pronounced ‘Cap delta’, incremental change. Micro, a millionth or 1e-6, or voltage amplification factor (‘A’ for valves), or Permeability (for magnetics). Permeability of free space. Omega, angular frequency – 2πf, originally called ‘pulsatance’. ‘Cap Omega’, commonly Ohm, unit of resistance. Degree(s). Positive, add, plus. Negative, subtract, minus. Approximately equal to. Less than. Greater than. Into. Less than, or equal to. Greater than, or equal to. Minus infinity, a nominal term used to indicate reduction of a signal to inaudibility.

Index A Abuse damage 377 AC mains 3-phase 242 Connectors 350 Fuses and breakers 253-255, 349 Impedance 347 Interaction via 355 Operable range testing 317 Ring 352 Spur 353 Voltage drop 346 Wiring recommendations 352-354 AC mains voltages Worldwide categories 342 Acoustic noise 328 Active cabinets 20 Active probing 394 Active systems Benefits 30 Bi-wiring 31 Adverse loads Protection 200-210 Table of 325 AES power rating 44 Air pollution 373 Airflow conventions 372 Amplifier Fitment 371 Test, warm-up 267 Wattage needed 333 Amplifier test preconditioning 266 Audio 2

B Baker clamp diode 181 Balance of gains 275 Balanced input 59-63, 356-360 Bandwidth 272, 273 Bass qualities 15 Batteries 86, 235

Beta 89, 90, 172 Biasing 88, 91, 98-99, 121, 127, 134-136 After repair 387, 388 Bipolar Junction Transistors: see BJTs BJTs 171 Cob 190 Compound 100 Leakage 175 Load Limits 194 Selection 176 Sharing 175 SOA 173 Voltage ratings 172 Breakthrough signal 283 Bridge Full 115 Modes 114 Rectifier 234 Bridged-bridge 117 Bridging 116, 336

C Cascaded complementary pairs 93 Cascades 112, 113 Cascading 112, 113 Cascoding 103, 104, 112, 113 Channel separation 283 Class A 118 Bias regulation 122 Efficiency 119 Class A-B 127 Efficiency 131 Class A/H 142 Class B 127 Class comparison 153 Class D 147 Class examples 154 Class G 138 Class G and H Efficiency 144

Index Class H 141 Benefits 145 Classes 84 Clip indication 213-215, 340 Testing 322 Clip indicators 214 Clipping 79, 186, 213 Prevention 79 Symmetry testing 307 CMR Measurement 285-286 See also Common Mode Rejection CMR vs. frequency 284, 285 Common mode distortion 51 Common Mode Rejection 60-63 Common-source 112 Common-source topology 112 Complementary push-pull 92 Compression driver 22 Computer control 80 Conduction angle 129 Cone driver 21 Constant voltage operation 228 Cooling requirements 222 Critical layout 191 Critical nodes 192 Crossover distortion 97, 128 Crossovers Active 29 Passive 28, 35, 38 Crosstalk 283 Current burst, testing 312-313 Current clipping 186 Current delivery 46, 193-211 Current feedback 160 Current mirror loading 104 Current-dumping 132

D Daisy chaining 56 Damage, mechanical 378 Damping control 161 Damping factor 36, 278 Damping surface 37 Darlington sub-topology 89

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DC Autonulling 67 Blocking capacitors 32, 65 Fault protection 219 Fuses 211 Input protection testing 322 Management strategies 66 Nulling 388 Offset, at output 218 Protection capacitors 32 Servo 67 Design defects 340 Design loop 38 DIM% testing 302 Diode Biasing 91, 97 Testing 397 Direct-coupling 65-68 Distortion, complex 9 Distortion tests 293-303 Double blind listening tests 316 Drive units 21 Dual power rails 102 Dynamic accuracy 12, 310 Dynamic headroom 137 Dynamic range 2 Testing 308 Dynamics Adjectives 12 HF 16

E Early effect 172, 175 Earthing RF 345 Safety 344 Efficiency, in perspective 6, 248, 251-252 EMC 329 EMI 61, 62, 150, 235, 329 Energy control 36 Energy storage, mechanical 34 Equal-loudness contours 7 Error feedback 163 Error takeoff 162 Error-correction 164 ESL (Electrostatic Loudspeaker) 24, 270

Index F

I

Frequency response 272 Fuses 211, 254 Thermal modulation 213

I2R losses 170, 312 IC drivers 109-113 ICs 388 Power 108 Protection 208 IGBT 183 IMD 298 3 tone method 302 CCIF testing 302 SMPTE testing 301 IMD% testing 300 Impedance, of loudspeakers 33, 194-196 Impulse response, testing 310 Inductive coupling 25 Infrasonic frequencies 15, 63 Input Damage protection 68 Harmful conditions 69 Protection strategies 70 RF filtration 58 Input cabling 361 Input connections Phono 356 XLR 357 Input connectors Balanced polarity 359 DIN 358 Jacks 358 Misc types 359 Input impedance 52, 286 High 54 Low 54 Vs. frequency 56 Variation 57 Input sensitivity 49 Input wiring, quasi balanced 360 Input-Output Comparator 214 Inrush current 237 Testing 318 Instantaneous dissipation 197 Insulated Gate Bipolar Transistors 183 Insulation 169, 223 Intermodulation distortion 298 (See IMD) Interstage coupling 89 IOC 214

G Gain 50-52, 274 Gain control 71 Compression 78 Motorised 75 Remote 74 Repeatability 78 using J-FETs 76 using LED+LDRs 76 using M-DACs 77 using Relays 77 using VCAs 75 Gain control scales 73 Gain pot 56, 71 Geometry 174 Germanium transistors 87, 393 Ground bounce 153 Groundlifting 345

H Harmonic distortion 286-297 Acceptable levels 288 Causes 287 Harmonic qualities 292 Harmonic spectra 299 Example 291 Harmonics 289 Individual analysis 297 Individual, example 299 Table of 290 Headroom metering 215 Hearing Sensitive 4 Skilled 5 Heat exchange 224, 338, 370 Heatsinks 222-226, 338, 385 Hfe testing 176, 395 Hi-Fi standards 259 High power Rationale 44, 332, 335

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Index L Large signal Conditions 184 Protection testing 215, 323 Lateral heatsink 338 Light bulb, as protector 391 Lin Later 103 Topology 96 Line-driver amplifiers 55 Listening tests 315, 336 Load Resistive 32-34, 268 Load-line limiter 2 slope 203 3 slope 204 5 level 205 Characteristics 202 Local feedback 159 Logical systems 80-81, 228 Loudspeaker Burnout 43, 44-45, 213 Configurations 27 Efficiencies 26 Parallel connection of 47, 48 Vulnerabilities 43 LTP 98 Cascaded 107 Double 106 Triple 106 LVD 169

M Mains current draw testing 320 Mechanical design 339 Micro-CAP simulation 195 Modes 84 Modes of control 155 Monoblock 19 MOSFETs 103, 135, 177, 207 Bandwidth 179 Biasing 180 Disadvantages 182 History 177 LMOS 178

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Protection 207 Ruggedness 179 Testing 395 Multi-channel 19 Music fundamentals 8, 289-292 Musical Asymmetry 14, 91, 307 Transients 14 Muting 221 Muting behaviour Evaluation 328

N Negative feedback Advantages and limitations 157 Modes 156 Nested feedback 159 Noding 193, 353 Noise specification 281 Noise spectra 282 Non-switching A-B 133 NSB 133

O Op-amp topologies 110 OPN 215-216, 312 Servicing 382 OTL 91, 228 Output capacitor 98, 229 Output connections 362-370 Output connectors 4mm posts 363 4mm sockets 364 EP series 367 Speakon 365 XLR 365 Output current capability 47, 210 Testing 312 Output fuse 212 Output interface 220 Output networks 215, 312 Output polarity 370 Output transformers 228 Overheating 375

Index P Paralleling, output stage 94, 175 Passive crossovers, impedance of 39 PCM 152 Periodic checks 378 PFC 237, 348 Phase linearity 67, 280 Phase Modulation Control 151 Phase response 278 Phase response matching 279 Pi-mode class 124 Piezo tweeter 24 Pinfin 225 Pink noise 264 Power, at low impedance 47 Power amplifiers Efficiency summary 252 Model specification 260 Standards 259 Power capability at HF 46 at LF 46 Dynamic 306 Testing 304-307 Power capability requirements 42 Power delivery, testing 304 Power Factor Correction 237, 348 Power ICs 108 Power ratings of drive units 41, 332 Power supply Adaptive 238 Efficiency 248, 251 Flyback 246 for bridged OPS 242 for testing 392 Fuse location 255 Fusing 253 Higher adaptive 250 History 231 PWM 243 Regulated 239 Resonant 247 SMPS 244 Powercon, connector 351

Pre-purchase recommendations 334 for pro users 335 Programme asymmetry 14, 91, 98 Programme ratings 332 Protection circuits 200-213, 227, 325-327 Prudent use 374 Pseudo-random signals 263 Pulse Code Modulation 152 Push-Pull, Transformer-coupled 89 PWM 148, 243

Q Q-A 341 Quality 3, 331 Quasi Complementary Output stage 94

R RAEF topology 101 Random signals, pseudo 263, 305 Rated power 304 Testing 305 RCCBs 346 Reactive load Survival testing 327 Rectifier diodes 234, 236 Regulated supplies 239 Reliability tips 337 Repair Observations 379 Reservoir capacitors 234, 237 Resonant PSU 248 RF Instability 190, 217, 400 Interference 62, 324 Protection 217, 322 Protection testing 323, 324, 329 Stability 191 Ribbon driver 23 Rise time, testing 308

S Safety earthing 344 Sample rate 149 Second-breakdown 174, 179

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Index Secondary crossover distortion 130 Sensitivity, of speakers 25, 333 Series mode NFB 156 Settling time test 312 Shielding effectiveness 61, 361 Shoot through 151 Short-Circuit, withstand testing 326 Signal present indication testing 321 Signal source, loading 55 Silicon transistors 95 Sinclair 108, 147, 305 Sinewaves 261 Multitone 262 Single-ended 88, 114 Skin effect 170, 369 Slew limit 185 Increasing 189 Testing 309 Slew limits vs. power 188 Slewing, musical causes 187 Sliding bias 123 SOA curve 173, 194, 199 Soft dome driver 22 Soft start 237 Testing 319 Soft-Clip 79 Sonic quality, adjectives 11 Sound, effortless 45 Sound reproduction 3 Space, adjectives 13 Spares sourcing 383 Speaker cable gauge table 368 Speaker cable shortcomings 369 Speaker cabling 367 Speaker impedance setting 370 Speakon connection table 366 Specifications, environmental 258 Squarewaves 261, 309-311 Stasis 126, 406 Sub-sonic filtering 64 Sub-sonic protection 63 Sub-topologies 87, 89, 91, 94, 98, 99, 100, 102 Super Class A 126 Super-Nova topology 111

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Supply rails, sharing 241 Surge, at switch on 237, 318 SWAMP 148 Symbols, for topologies 86 Symmetrical topologies 105 Sziklai 90

T Tap changing, transformer 343 Test equipment AC mains, measurement 265 Computer driven 264 Test load 268, 392 For ESL 270 Speaker simulation 271 Test load specifications 269 Testing environment 265 THD 293 THD% Bandwidth 295 Test level 295 vs. frequency 294 vs. level 296 Thermal protection 226 Testing 327 Tonal qualities 10 Tone-bursts 263 Topologies 83 Symbols 86 Torquing 385 Total Harmonic Distortion 293 Transformer, mains 233, 235, 343 Transient response 273 Testing 311 Transistor, BJT testing 393 Transistor testing 382 Transistors, Equivalence 384 Triple, QUAD 101 Troubleshooting 374-375

U Ultrasonic Content 16 Digital limits 17

Index V V-I limiter 200-206 Limitations 209 Mapping capability 210 V-I limits 194 V-I output capability 198 V-I protection Current sampling 206 Evolution 201 Shared sensing 206 V-I testing Baxandall method 211 Variac 266, 343, 390 Vbe multiplier 99 Voice coil 23 Voltage gain 50, 274 Voltage matching 53 Voltage sources 40, 157, 276

Z Zener testing 398 Zero feedback 158 Zero-crossing 128 Zobel networks 32, 216, 382

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